Web Platform Design Principles

1. Principles behind design of Web APIs

The Design Principles are directly informed by the ethical framework set out in the Ethical Web Principles [ETHICAL-WEB]. These principles provide concrete practical advice in response to the higher level ethical responsibilities that come with developing the web platform.

1.1. Put user needs first (Priority of Constituencies)

If a trade-off needs to be made, always put user needs above all.

Similarly, when beginning to design an API, be sure to understand and document the user need that the API aims to address.

The internet is for end users: any change made to the web platform has the potential to affect vast numbers of people, and may have a profound impact on any person’s life. [RFC8890]

User needs come before the needs of web page authors, which come before the needs of user agent implementors, which come before the needs of specification writers, which come before theoretical purity.

Like all principles, this isn’t absolute. Ease of authoring affects how content reaches users. User agents have to prioritize finite engineering resources, which affects how features reach authors. Specification writers also have finite resources, and theoretical concerns reflect underlying needs of all of these groups.

See also:

• The web should not cause harm to society

• The web must enhance individuals' control and power

• [RFC8890]

1.2. It should be safe to visit a web page

When adding new features, design them to preserve the user expectation that visiting a web page is generally safe.

The Web is named for its hyperlinked structure. In order for the web to remain vibrant, users need to be able to expect that merely visiting any given link won’t have implications for the security of their computer, or for any essential aspects of their privacy.

For example, an API which allows any website to detect the use of assistive technologies may make users of these technologies feel unsafe visiting unknown web pages, since any web page may detect this private information.

If users have a realistic expectation of safety, they can make informed decisions between Web-based technologies and other technologies. For example, users may choose to use a web-based food ordering page, rather than installing an app, since installing a native app is riskier than visiting a web page.

To work towards making sure the reality of safety on the web matches users' expectations, we can take complementary approaches when adding new features:

• We can improve the user interfaces through which the Web is used to make it clearer what users of the Web should (and should not) expect;

• We can change the technical foundations of the Web so that they match user expectations of privacy;

• We can consider the cases where users would be better off if expectations were higher, and in those cases try to change both technical foundations and expectations.

A new feature which introduces safety risks may still improve user safety overall, if it allows users to perform a task more safely on a web page than it would be for them to install a native app to do the same thing. However, this benefit needs to be weighed against the common goal of users having a reasonable expectation of safety on web pages.

See also:

• Security and Privacy Self-Review

1.3. Trusted user interface should be trustworthy

Consider whether new features impact trusted user interfaces.

Users depend on trusted user interfaces such as the address bar, security indicators and permission prompts, to understand who they are interacting with and how. These trusted user interfaces must be able to be designed in a way that enables users to trust and verify that the information they provide is genuine, and hasn’t been spoofed or hijacked by the website.

If a new feature allows untrusted user interfaces to resemble trusted user interfaces, this makes it more difficult for users to understand what information is trustworthy.

For example, JavaScript alert() allows a page to show a modal dialog which looks like part of the browser. This is often used to attempt to trick users into visiting scam websites. If this feature was proposed today, it would probably not proceed.

1.4. Ask users for meaningful consent when appropriate

If a useful feature has the potential to cause harm to users, make sure that the user can give *meaningful consent* for that feature to be used, and that they can refuse consent effectively.

In order to give meaningful consent, the user must:

• understand what permission they may choose whether to grant the web page

• be able to choose to give or refuse that permission effectively.

If a feature is powerful enough to require a user consent, but it’s impossible to explain to a typical user what they are consenting to, that’s a signal that you may need to reconsider the design of the feature.

If a permission prompt is shown, and the user doesn’t grant permission, the Web page should not be able to do anything that the user believes they have refused consent for.

By asking for consent, we can inform the user of what capabilities the web page does or doesn’t have, reinforcing their confidence that the web is safe. However, the user benefit of a new feature must justify the additional burden on users to decide whether to grant permission for each feature whenever it’s requested by a Web page.

For example, the Geolocation API grants access to a user’s location. This can help users in some contexts, like a mapping application, but may be dangerous to some users in other contexts - especially if used without the user’s knowledge. So that the user may decide whether their location may be used by a Web page, a permission prompt should be shown to the user asking whether to grant location access. If the user refuses permission, no location information is available to the Web page.

See also:

• Security and privacy are essential

1.5. Support the full range of devices and platforms (Media Independence)

As much as possible, ensure that features on the web work across different input and output [devices, screen sizes, interaction modes, platforms, and media] (https://www.w3.org/2001/tag/doc/ethical-web-principles/#multi).

One of the main values of the Web is that it’s extremely flexible: a Web page may be viewed on virtually any consumer computing device at a very wide range of screen sizes, may be used to generate printed media, and may be interacted with in a large number of different ways. New features should match the existing flexibility of the web platform.

Features should also be designed so that the easiest way to use them maintains flexibility.

1.6. Leave the web better than you found it

As you add new capabilities to the web platform, do so in a way that improves the overall platform, for example its security or privacy vulnerabilities, or accessibility characteristics. The existence of a defect in one part of the platform must not be used as a license for adding or extending such defect into new capabilities and thereby further decreasing the overall platform quality. Where possible, build new web capabilities that improve the overall platform quality.

Parts of the web platform evolve independently. Issues that are present with a certain web technology now may be fixed in a subsequent iteration. Duplicating these issues makes fixing them more difficult. By adhering to this principle we can make sure overall platform quality improves over time.

2. API Design Across Languages

2.1. Prefer simple solutions

Look hard for simple solutions to the user needs you intend to address.

Simple solutions are generally better than complex solutions, although they may be harder to find. Simpler features are easier for user agents to implement and test, more likely to be interoperable, and easier for authors to understand. It is especially important to design your feature so that the most common use cases are easy to accomplish.

Make sure that your user needs are well-defined. This allows you to avoid scope creep, and make sure that your API does actually meet the needs of all users. Of course, complex or rare use cases are also worth solving, though their solutions may be more complicated to use. As Alan Kay said, "simple things should be simple, complex things should be possible."

Do note however that while common cases are often simple, commonality and complexity are not always correlated.

See also:

• [LEAST-POWER]

2.2. Consider tradeoffs between high level and low level APIs

High-level APIs allow user agents more ability to intervene in various ways on behalf of the user, such as to ensure accessibility, privacy, or usability.

Low-level APIs afford authors room for experimentation so that high level APIs can organically emerge from usage patterns over time. They also provide an escape hatch when the higher-level API is not adequate for the use case at hand.

Lower level building blocks cannot always be exposed as Web APIs. A few possible reasons for this are to preserve the user’s security and privacy, or to avoid tying Web APIs to specific hardware implementations. However, high level APIs should be designed in terms of building blocks over lower level APIs whenever possible. This may guide decisions on how high level the API needs to be.

A well-layered solution should ensure continuity of the ease-of-use vs power tradeoff curve and avoid sharp cliffs where a small amount of incremental use case complexity results in a large increase of code complexity.

2.3. Name things thoughtfully

Name APIs with care. Naming APIs well makes it much easier for authors to use them correctly.

See the more detailed Naming principles section for specific guidance on naming.

2.4. Be consistent

It is good practice to consider precedent in the design of your API and to try to be consistent with it.

There is often a tension between API ergonomics and consistency, when existing precedent is of poor usability. In some cases it makes sense to break consistency to improve usability, but the improvement should be very significant to justify this.

Since the web platform has gradually evolved over time, there are often multiple conflicting precedents which are mutually exclusive. You can weigh which precedent to follow by taking into account prevalence (all else being equal, follow the more popular precedent), API ergonomics (all else being equal, follow the more usable precedent), and API age (all else being equal, follow the newer precedent).

There is often a tension between internal and external consistency. Internal consistency is consistency with the rest of the system, whereas external consistency is consistency with the rest of the world. In the web platform, that might materialize in three layers: consistency within the technology the API belongs to (e.g. CSS), consistency with the rest of the web platform, and in some cases external precedent, when the API relates to a particular specialized outside domain. In those cases, it is useful to consider what the majority of users will be. Since for most APIs the target user is someone who is familiar with the technology they are defined in, err on the side of favoring consistency with that.

There is also a separate section on naming consistency.

2.5. New features should be detectable

Provide a way for authors to programmatically detect whether your feature is available, so that web content may gracefully handle the feature not being present.

An existing feature may not be available on a page for a number of reasons. Two of the more common reasons are because it hasn’t been implemented yet, or because it’s only available in secure contexts.

Authors shouldn’t need to write different code to handle each scenario. That way, even if an author only knows or cares about one scenario, the code will handle all of them.

When a feature is available but isn’t feasible to use because a required device isn’t present, it’s better to expose that the feature is available and have a separate way to detect that the device isn’t. This allows authors to handle a device not being available differently from the feature not being available, for example by suggesting the user connect or enable the device.

See § 9.2 Use care when exposing APIs for selecting or enumerating devices.

Authors should always be able to detect a feature from JavaScript, and in some cases the feature should also be detectable in the language where it’s used (such as @supports in CSS).

In some cases, it may not be appropriate to allow feature detection. Whether the feature should be detectable or not should be based on the user need for the feature: if there is a user need or design principle which would fail if feature detection were available for the feature, then you should not support feature detection.

Also, if a feature is generally not exposed to developers, it is not appropriate to support feature detection. For example, private browsing mode is a concept which is recognised in web specifications, but not exposed to authors. For private browsing mode to support the user’s needs, it must not be feature detected.

See also:

• § 2.7 Don’t reveal that private browsing mode is engaged

• § 2.9 Don’t reveal that assistive technologies are being used

• § 2.6 Consider limiting new features to secure contexts

2.6. Consider limiting new features to secure contexts

Always limit your feature to secure contexts if it would pose a risk to the user without the authentication, integrity, or confidentiality that’s present only in secure contexts.

For other features, TAG members past and present haven’t reached consensus on general advice. Some believe that all new features (other than features which are additions to existing features) should be limited to secure contexts. This would help encourage the use of HTTPS, helping users be more secure in general.

Others believe that features should only be limited to secure contexts if they have a known security or privacy impact. This lowers the barrier to entry for creating web pages that take advantage of new features which don’t impact user security or privacy.

2.7. Don’t reveal that private browsing mode is engaged

Make sure that your feature doesn’t give authors a way to detect private browsing mode.

Some people use private browsing mode to protect their own personal safety. Because of this, the fact that someone is using private browsing mode may be sensitive information about them. This information may harm people if it is revealed to a web site controlled by others who have power over them (such as employers, parents, partners, or state actors).

Given such dangers, websites should not be able to detect that private browsing mode is engaged.

See also:

• Security and privacy are essential

• What data does this specification expose to an origin?

• § 2.8 Consider how your API should behave in private browsing mode

2.8. Consider how your API should behave in private browsing mode

If necessary, specify how your API should behave differently in private browsing mode.

For example, if your API would reveal information that would allow someone to correlate a single user’s activity both in and out of private browsing mode, consider possible mitigations such as introducing noise, or using permission prompts to give the user extra information to help them meaningfully consent to this tracking (see § 1.4 Ask users for meaningful consent when appropriate).

Private browsing modes enable users to browse the web without leaving any trace of their private browsing on their device. Therefore, APIs which provide client-side storage should not persist data stored while private browsing mode is engaged after it’s disengaged. This can and should be done without revealing any detectable API differences to the site.

See also:

• § 2.7 Don’t reveal that private browsing mode is engaged

• Does this specification introduce new state for an origin that persists across browsing sessions?

• Security and privacy self review: Private Browsing

2.9. Don’t reveal that assistive technologies are being used

Make sure that your API doesn’t provide a way for authors to detect that a user is using assistive technology without the user’s consent.

The web platform must be accessible to people with disabilities. If a site can detect that a user is using an assistive technology, that site can deny or restrict the user’s access to the services it provides.

People who make use of assistive technologies are often vulnerable members of society; their use of assistive technologies is sensitive information about them. If an API provides access to this information without the user’s consent, this sensitive information may be revealed to others (including state actors) who may wish them harm.

Sometimes people propose features which aim to improve the user experience for users of assistive technology, but which would reveal the user’s use of assistive technology as a side effect. While these are well intentioned, they violate § 1.2 It should be safe to visit a web page, so alternative solutions must be found.

See also:

• Web Technology Accessibility Guidelines

• Security and privacy are essential

• What data does this specification expose to an origin?

2.10. Require user activation for powerful APIs

Some powerful APIs can produce intrusive UI (eg. auto-playing audio), expose user data (eg. interacting with the clipboard), perform a background activity without an obvious indicator to the user (eg. accessing local storage), or prompt the user to interact with trusted UI (eg. permission prompts, device hardware features). These APIs should be designed to require some indication of user intention (such as user activation) in order to function. This indicates that the user is intentionally interacting with the web page in question.

User activation is defined in detail in the HTML standard. You should think about the effect your API has on the user experience, as well as any risks presented to the user, when deciding whether user activation needs to only occur once overall (sticky), periodically (transient) or once per API call (transient consuming).

Note that while user activation is in many cases necessary, it is not always sufficient to protect users from invasive behaviours, and seeking meaningful consent is also important.

2.11. Support non-fully active BFCached documents

Specify how your feature behaves in non-fully active BFCached (Back/forward cached) documents if possible.

If your feature does anything that fall into the following categories:

• Interacts with a document from the "outside" (e.g. sends information to a document)

• Makes cross-document interaction/resource sharing possible (e.g. holding locks)

• May malfunction when a document is kept in a non-fully active BFCached state (instead of getting destroyed) after the user navigates away from it or gets restored (e.g. expects that a state saved in the document won’t span multiple navigations)

You should specify how it works with non-fully active BFCached documents, following the guidelines in the Supporting BFCached Documents guide.

Note: It is possible for a document to become non-fully active for other reasons not related to BFcaching, such as when the iframe holding the document gets detached. This guideline only focuses on the case where BFCache is involved, and not other cases that might cause a document to become non-fully active.

2.12. Prioritize usability over compatibility with third-party tools

Design new features with usability as the primary goal, and compatibility with third-party tooling as a secondary goal.

The web platform benefits from a wide ecosystem of tooling to facilitate easier and faster development. A lot of the time, the syntax of an upcoming web platform feature may conflict with that of a third-party tool causing breakage. This is especially common as third-party tools are often used to prototype new web platform features.

In general, web platform features last a lot longer than most third-party tools, and thus giving them the optimal syntax and functionality should be of high priority.

In some cases, the conflict will introduce problems across a large number of web sites, necessitating the feature’s syntax to be redesigned to avoid clashes.

However, these cases should be exceptions.

When deciding whether to break third party tools with new syntax, there are several factors to consider, such as severity of the breakage, popularity of the third party tool, and many more.

Possibly the most important factor is how severely would the usability of the web platform feature be compromised if its syntax was changed to avoid breaking the third party tool? If several alternatives of similar usability are being considered, it is usually preferable to prioritize the ones that inconvenience third party tools the least.

However, if avoiding breaking the third party tool would lead to a significant negative impact on of the feature’s usability, that is rarely an acceptable tradeoff, unless it causes significant breakage of live websites.

Languages should also provide mechanisms for extensibility that authors can use to extend the language without breaking future native functionality, to reduce such dilemmas in the future.

3. HTML

This section details design principles for features which are exposed via HTML.

3.1. Re-use attribute names (only) for similar functionality

If you are adding a feature that is specified through an HTML attribute, check if there is an existing attribute name on another element that specifies similar functionality. Re-using an existing attribute name means authors can utilize existing knowledge, maintains consistency across the language, and keeps its vocabulary small.

If you do re-use an existing attribute, try to keep its syntax as close as possible to the syntax of the existing attribute.

The inverse also applies: do not re-use an existing attribute name if the functionality you are adding is not similar to that of the existing attribute.

3.2. Use space-separated attributes for short lists of values, separate elements for longer lists

When specifying metadata about an element that can be a list of values, common practice is to use a space-separated list and expose it as a DOMTokenList.

Consistency with other parts of the Web Platform is important, even if this means using another character to separate values.

Regardless of syntax, attributes should only be used for short lists of values. For longer lists, embedding the entire list in an attribute is discouraged. Instead, current practice is to use separate elements to represent the list items (and any metadata about them). These elements could either be children of the element in question, or linked through an attribute.

In rare instances, other tradeoffs are necessary.

3.3. Do not pause the HTML parser

Ensure that your design does not require HTML parser to pause to handle external resources.

As a page parses, the browser discovers assets that the page needs, and figures out a priority in which they should be loaded in parallel. Such parsing can be disrupted by a resource which blocks the discovery of subsequent resources. At worst, it means the browser downloads items in series rather than parallel. At best, it means the browser queues downloads based on speculative parsing, which may turn out to be incorrect.

Features that block the parser generally do so because they want to feed additional content into the parser before subsequent content. This is the case of legacy <script src="…"> elements, which can inject into the parser using document.write(…). Due to the performance issues above, new features must not do this.

3.4. Avoid features that block rendering

Features that require resource loading or other operations before rendering the page, often result in blank page (or the previous page). The result is a poor user experience.

Consider adding such features only in cases when the overall user experience is improved. A canonical example of this is blocking rendering in order to download and process a stylesheet. The alternative user experience is a flash of unstyled content, which is undesirable.

3.5. Keep attributes in sync

New content attributes should have a corresponding IDL attribute with the same name, and the state between the two should be kept synchronized. Carving out a synchronized IDL attribute with inconsistent naming results in confusion, and should be avoided.

3.6. Name URL-containing attributes based on their primary purpose

If the element enables the user to navigate to the URL contained in the attribute, call the attribute href, like the a element’s href attribute.

Note: In hindsight, form's action attribute should have been named href.

If the element causes the resource at the given URL to be loaded, call the attribute src, like the img element’s src attribute or the script element’s src attribute.

Note: HTML has a number of legacy inconsistencies that should not be emulated, like the way link's href attribute might allow for navigation or for resource loading, depending on the value of the element’s rel attribute.

If the attribute identifies a URL that is auxilliary to the element’s purpose, like video's poster, q's cite, or a’s ping, name the attribute after its semantics.

Note: remember that attributes containing URLs should be represented in IDL as USVString; see § 8.2 Represent strings appropriately.

4. Cascading Style Sheets (CSS)

This section details design principles for features which are exposed via CSS.

4.1. Separate CSS properties based on what should cascade separately

Decide which values should be grouped together as CSS properties and which should be separate properties based on what makes sense to set independently.

CSS cascading allows declarations from different rules or different style sheets to override one another. A set of values that should all be overridden together should be grouped together in a single property so that they cascade together. Likewise, values that should be able to be overridden independently should be separate properties.

4.2. Make appropriate choices for whether CSS properties are inherited

Decide whether a property should be inherited based on whether the effect of the property should be overridden or added to if set on an ancestor as well as a descendant.

If setting the property on a descendant element needs to override (rather than add to) the effect of setting it on an ancestor, then the property should probably be inherited.

If setting the property on a descendant element is a separate effect that adds to setting it on an ancestor, then the property should probably not be inherited.

A specification of an non-inherited property requiring that the handling of an element look at the value of that property on its ancestors (which may also be slow) is a "code smell" that suggests that the property likely should have been inherited. A specification of an inherited property requiring that the handling of an element ignore the value of a property if it’s the same as the value on the parent element is a "code smell" that suggests that the property likely should not have been inherited.

4.3. Choose the computed value type based on how the property should inherit

Choose the computed value of a CSS property based on how it will inherit, including how values where it depends on other properties should inherit.

Inheritance means that an element gets the same computed value for a property that its parent has. This means that processing steps that happen before reaching the computed value affect the value that is inherited, and those that happen after (such as for the used value) do not.

<body style="font-size: 20px; line-height: 1.4">

  <p>This body text has a line height of 28px.</p>

  <h2 style="font-size: 200%">
    This heading has a line-height of 56px,
    not 28px, even though the line-height was declared on the body.
    This means that the 40px font won’t overflow the line height.
  </h2>
</body>

See also:

• Computed Values Patterns

4.4. Name CSS properties and values appropriately

The names of CSS properties are usually nouns, and the names of their values are usually adjectives (although sometimes nouns).

Words in properties and values are separated by hyphens. Abbreviations are generally avoided.

Use the root form of words when possible rather than a form with a grammatical prefix or suffix (for example, "size" rather than "sizing").

The list of values of a property should generally be chosen so that new values can be added. Avoid values like yes, no, true, false, or things with more complex names that are basically equivalent to them.

Avoid words like "mode" or "state" in the names of properties, since properties are generally setting a mode or state.

See § 12 Naming principles for general (cross-language) advice on naming.

4.5. Content should be viewable and accessible by default

Design CSS properties or CSS layout systems (which are typically values of the display property), to preserve the content as viewable, accessible and usable by default.

5. JavaScript Language

5.1. Use JavaScript for Web APIs

When designing imperative APIs for the Web, use JavaScript. In particular, you can freely rely upon language-specific semantics and conventions, with no need to keep things generalized.

5.2. Preserve run-to-completion semantics

Don’t modify data accessed via JavaScript APIs while a JavaScript event loop is running.

A JavaScript Web API is generally a wrapper around a feature implemented in a lower-level language, such as C++ or Rust. Unlike those languages, when using JavaScript developers can expect that once a piece of code begins executing, it will continue executing until it has completed.

Because of that, JavaScript authors take for granted that the data available to a function won’t change unexpectedly while the function is running.

So if a JavaScript Web API exposes some piece of data, such as an object property, the user agent must not update that data while a JavaScript task is running. Instead, if the underlying data changes, queue a task to modify the exposed version of the data.

5.3. Don’t expose garbage collection

Ensure your JavaScript Web APIs don’t provide a way for an author to know the timing of garbage collection.

The timing of garbage collection is different in different user agents, and may change over time as user agents work on improving performance. If an API exposes the timing of garbage collection, it can cause programs to behave differently in different contexts. This means that authors need to write extra code to handle these differences. It may also make it more difficult for user agents to implement different garbage collection strategies, if there is enough code which depends on timing working a particular way.

This means that you shouldn’t expose any API that acts as a weak reference, e.g. with a property that becomes null once garbage collection runs. Object and data lifetimes in JavaScript code should be predictable.

getElementsByTagName gives no sign that it may depend on the timing of garbage collection. In contrast, APIs which are explicitly designed to depend on garbage collection, like WeakRef or FinalizationRegistry, set accurate author expectations about the interaction with garbage collection.

6. JavaScript API Surface Concerns

6.1. Attributes should behave like data properties

[WEBIDL] attributes should act like simple JavaScript object properties.

In reality, IDL attributes are implemented as accessor properties with separate getter and setter methods. To make them act like JavaScript object properties:

• Getters must not have any observable side effects.

• Getters should not perform any complex operations.

• Ensure that obj.attribute === obj.attribute is always true. Don’t create a new value each time the getter is called.

• If possible, ensure that given obj.attribute = x, obj.attribute === x is true. (This may not be possible if some kind of conversion is necessary for x.)

If you were thinking about using an attribute, but it doesn’t behave this way, you should probably use a method instead.

6.2. Consider whether objects should be live or static

If an API gives access to an object representing some internal state, decide whether that object should continue to be updated as the state changes.

An object which represents the current state at all times is a live object, while an object which represents the state at the time it was created is a static object.

Live objects

If an object allows the author to change the internal state, that object should be live. For example, DOM Nodes are live objects, to allow the author to make changes to the document with an understanding of the current state.

Properties of live objects may be computed as they are accessed, instead of when the object is created. This makes live objects sometimes a better choice if the data needed is complex to compute, since there is no need to compute all the data before the object is returned.

A live object may also use less memory, since there is no need to copy data to a static version.

Static objects

If an object represents a list that might change, most often the object should be static. This is so that code iterating over the list doesn’t need to handle the possibility of the list changing in the middle.

let list = document.getElementsByTagName("td");

for (let i = 0; i < list.length; i++) {
    let td = list[i];
    let tr = document.createElement("tr");
    tr.innerHTML = td.outerHTML;

    // This has the side-effect of removing td from the list,
    // causing the iteration to become unpredictable.
    td.parentNode.replaceChild(tr, td);
}

Note: For maplike and setlike types, this advice may not apply, since these types were designed to behave well when they change while being iterated.

If it would not be possible to compute properties at the time they are accessed, a static object avoids having to keep the object updated until it’s garbage collected, even if it isn’t being used.

If a static object represents some state which may change frequently, it should be returned from a method, rather than available as an attribute.

See also:

• § 6.1 Attributes should behave like data properties

6.3. Use attributes or methods appropriately

Sometimes it is unclear whether to use an attribute or a method.

• Attribute getters must not have any (observable) side effects. If you have expected side effects, use a method.

• Attribute getters should not perform any blocking operations. If a getter requires performing a blocking operation, it should be a method.

• If the underlying object has not changed, attribute getters should return the same object each time it is called. This means obj.attribute === obj.attribute must always hold. Returning a new value from an attribute getter each time is not allowed. If this does not hold, the getter should be a method.

Note: An antipattern example of a blocking operation is with getters like offsetTop performing layout.

For attributes, whenever possible, preserve values given to the setter for return from the getter. That is, given obj.attribute = x, a subsequent obj.attribute === x should be true. (This will not always be the case, e.g., if a normalization or type conversion step is necessary, but should be held as a goal for normal code paths.)

The object you want to return may be live or static. This means:

• If live, then return the same object each time, until a state change requires a different object to be returned. This can be returned from either an attribute or a method.

• If static, then return a new object each time. In which case, this should be be a method.

6.4. Prefer dictionary arguments over primitive arguments

API methods should generally use dictionary arguments instead of a series of optional primitive arguments.

This makes the code that calls the method much more readable. It also makes the API more extensible in the future, particularly if multiple arguments with the same type are needed.

new Event("example",
          { bubbles: true,
            cancelable: false})

new Event("example", true, false)

window.scrollBy({ left: 50, top: 0 })

window.scrollBy(50, 0)

The dictionary itself should be an optional argument, so that if the author is happy with all of the default options, they can avoid passing an extra argument.

element.scrollIntoView(false, {});

element.scrollIntoView(false);

See also:

• Hall of API Shame: Boolean Trap.

• APIs that have boolean arguments defaulting to true

• § 6.5 Make method arguments optional if possible

• § 6.6 Name optional arguments appropriately

6.5. Make method arguments optional if possible

If an argument for an API method has a reasonable default value, make that argument optional and specify the default value.

Note: Exceptions have been made for legacy interoperability reasons (such as XMLHttpRequest), but this should be considered a design mistake rather than recommended practice.

The API must be designed so that if an argument is left out, the default value used is the same as converting undefined to the type of the argument. For example, if a boolean argument isn’t set, it must default to false.

When deciding between different list data types for your API, unless otherwise required, use the following list types:

• Method list arguments should be of type sequence<T>

• Method return values should be of type sequence<T>

• Attributes should be of type ObservableArray<T>

See also:

• § 6.4 Prefer dictionary arguments over primitive arguments

6.6. Name optional arguments appropriately

Name optional arguments to make the default behavior obvious without being named negatively.

This applies whether they are provided in a dictionary or as single arguments.

See also:

• § 12 Naming principles

6.7. Classes should have constructors when possible

Make sure that any class that’s part of your API has a constructor, if appropriate.

By default, [WEBIDL] interfaces generate "non-constructible" classes: trying to create instances of them using new X() will throw a TypeError. To make them constructible, you can add appropriate constructor operation to your interface, and defining the algorithm for creating new instances of your class.

This allows JavaScript developers to create instances of the class for purposes such as testing, mocking, or interfacing with third-party libraries which accept instances of that class. It also gives authors the ability to create a subclass of the class, which is otherwise prevented, because of the way JavaScript handles subclasses.

This won’t be appropriate in all cases. For example:

• Some objects represent access to privileged resources, so they need to be constructed by factory methods which can access those resources.

• Some objects have very carefully controlled lifecycles, so they need to be created and accessed through specific methods.

• Some objects represent an abstract base class, which shouldn’t be constructed, and which authors should not be able to define subclasses for.

The Event class, and all its derived interfaces, are constructible. This is useful when testing code which handles events: an author can construct an Event to pass to a method which handles that type of event.

The Window class isn’t constructible, because creating a new window is a privileged operation with significant side effects. Instead, the window.open() method is used to create new windows.

The ImageBitmap class isn’t constructible, as it represents an immutable, ready-to-paint bitmap image, and the process of getting it ready to paint must be done asynchronously. Instead, the createImageBitmap() factory method is used to create it.

The DOMTokenList class is, sadly, not constructible. This prevents the creation of custom elements that expose their token list attributes as DOMTokenLists.

Factory methods can complement constructors, but generally should not be used instead of them. It may still be valuable to include factory methods in addition to constructors, when they provide additional benefits. A common such case is when an API includes base classes and multiple specialized subclasses, with a factory method for creating the appropriate subclass based on the parameters passed. Often the factory method is a static method on the closest common base subclass of the returned result.

6.8. Use synchronous when appropriate

Where possible, prefer synchronous APIs when designing a new API. Synchronous APIs are simpler to use, and need less infrastructure set-up (such as making functions async).

An API should generally be synchronous if the following rules of thumb apply:

• The API is not expected to ever be gated behind a permission prompt, or another dialog such as a device selector.

• The API implementation will not be blocked by a lock, filesystem or network access, for example, inter-process communication.

• The execution time is short and deterministic.

6.9. Design asynchronous APIs using Promises

If an API method needs to be asynchronous, use Promises, not callback functions.

Using Promises consistently across the web platform means that APIs are easier to use together, such as by chaining promises. Promise-using code also tends to be easier to understand than code using callback functions.

An API might need to be asynchronous if:

• the user agent needs to prompt the user for permission,

• some information might need to be read from disk, or requested from the network,

• the user agent may need to do a significant amount of work on another thread, or in another process, before returning the result.

See also:

• Writing Promise-Using Specifications

6.10. Cancel asynchronous APIs/operations using AbortSignal

If an asynchronous method can be cancelled, allow authors to pass in an AbortSignal as part of an options dictionary.

const controller = new AbortController();
const signal = controller.signal;
geolocation.read({ signal });

Using AbortSignal consistently as the way to cancel an asychronous operation means that authors can write less complex code.

For example, there’s a pattern of using a single AbortSignal for several ongoing operations, and then using the corresponding AbortController to cancel all of the operations at once if necessary (such as if the user presses "cancel", or a single-page app navigation occurs.)

Even if cancellation can’t be guaranteed, you can still use an AbortController, because a call to abort() on AbortController is a request, rather than a guarantee.

6.11. Use strings for constants and enums

If your API needs a constant, or a set of enumerated values, use string values.

Strings are easier for developers to inspect, and in JavaScript engines there is no performance benefit from using integers instead of strings.

If you need to express a state which is a combination of properties, which might be expressed as a bitmask in another language, use a dictionary object instead. This object can be passed around as easily as a single bitmask value.

6.12. Use properties and methods appropriately

Sometimes it is unclear whether to use a property or a method.

• Property getters must not have any (observable) side effects. If you have expected side effects, use a method.

• Property getters are expected to represent the state of a given object. This means a getter should be able to efficiently return a value using the existing state. If a getter does not satisfy this, it should be a method. (A notable failure of the platform in this regard is getters like offsetTop performing layout; do not repeat this mistake.)

• If the underlying object has not changed, property getters should return the same object each time it is called. This means obj.property === obj.property must always hold. Returning a new value from a property getter each time is not allowed. If this does not hold, the getter should be a method.

For properties, whenever possible, preserve values given to the setter for return from the getter. That is, given obj.property = x, a subsequent obj.property === x should be true. (This will not always be the case, e.g., if a normalization or type conversion step is necessary, but should be held as a goal for normal code paths.)

The object you want to return may be live or static. This means:

• If live, then return the same object each time, until a state change requires a different object to be returned. This can be returned from either an property or a method.

• If static, then return a new object each time. In which case, this should be be a method.

7. Event Design

7.1. Use promises for one time events

Follow the advice in the Writing Promise-Using Specifications guideline.

7.2. Events should fire before Promises resolve

If a Promise-based asynchronous algorithm dispatches events, it should dispatch them before the Promise resolves, rather than after.

This guarantees that once the Promise resolves, all effects of the algorithm have been applied. For example, if an author changes some state in reaction to an event which the Promise dispatches, they can be sure that all of the state is consistent if the Promise is resolved.

7.3. Don’t invent your own event listener-like infrastructure

When creating an API which allows authors to start and stop a process which generates notifications, use the existing event infrastructure to allow listening for the notifications. Create separate API controls to start/stop the underlying process.

See:

• § 7.5 Use events for notification

• § 7.8 Use Events and Observers appropriately

• § 7.7 Use plain Events for state

7.4. Always add event handler attributes

If your API adds a new event type, add a corresponding onyourevent event handler IDL attribute to the interface of any EventHandler which may handle the new event.

it’s important to continue to define event handler IDL attributes because:

• they preserve consistency in the platform

• they enable feature-detection for the supported events (see § 2.5 New features should be detectable)

For consistency, if the event needs to be handled by HTML and SVG elements, add the event handler IDL attributes on the GlobalEventHandlers interface mixin, instead of directly on the relevant element interface(s). Similarly, add event handler IDL attributes to WindowEventHandlers rather than Window.

7.5. Use events for notification

Events shouldn’t be used to trigger changes, only to deliver a notification that a change has already finished happening.

7.6. Guard against potential recursion

If your API includes a long-running or complicated algorithm, prevent calling into the algorithm if it’s already running.

If an API method causes a long-running algorithm to begin, you should use events to notify user code of the progress of the algorithm. However, the user code which handles the event may call the same API method, causing the complex algorithm to run recursively. The same event may be fired again, causing the same event handler to be fired, and so on.

To prevent this, make sure that any "recursive" call into the API method simply returns immediately. This technique is "guarding" the algorithm.

Note: A caution about early termination: if the algorithm being terminated would go on to ensure some critical state consistency, be sure to also make the relevant adjustments in state before early termination of the algorithm. Not doing so can lead to inconsistent state and end-user-visible bugs when implemented as-specified.

Note: Be cautious about throwing exceptions in early termination. Keep in mind the scenario in which developers will be invoking the algorithm, and whether they would reasonably expect to handle an exception in this [perhaps rare] case. For example, will this be the only exception in the algorithm?

You won’t always be able to "guard" in this way. For example, an algorithm may have too many entry-points to reliably check all of them. If that’s the case, another option is to defer calling the author code to a later task or microtask. This avoids a stack of recursion, but can’t avoid the risk of an endless loop of follow-up tasks.

Deferring an event is often specified as "queue a task to fire an event...".

You should always defer events if the algorithm that triggers the event could be running on a different thread or process. In this case, deferral ensures the events can be processed on the correct task in the task queue.

Both the "guarding" and the "deferring" approach have trade-offs.

"Guarding" an algorithm guarantees:

• at the time events are fired, there is no chance that the state may have changed between the guarded algorithm ending and the event firing.

• events fired during the algorithm, such as events to notify user code of a state change made as part of the algorithm, can be fired immediately, notifying code of the change without needing to wait for the next task.

• user code running in the event handler can observe relevant state directly on the instance object they were fired on, rather than needing to be given a copy of the relevant state with the event.

If the events are deferred instead:

• there is no guarantee that they will be first in the task queue once the algorithm completes.

• any other task may change the object’s state you should include any state relevant to the event with the deferred event.

  • This usually involves a new subclass of Event, with new attributes to hold the state.

    For example, the ProgressEvent adds loaded, total, etc. attributes to hold the state.

• if different parts of an algorithm need to coordinate, you may need to define an explicit state machine (well-defined state transitions) to ensure that when a deferred event fires, the behavior of inspecting or changing state is well-defined.

  For example, in [payment-request], the PaymentRequest's [[state]] internal slot explicitly tracks the object’s state through its well-defined transitions.

  • These state transitions often use the guarding technique themselves, to ensure the state transitions happen appropriately.

    For example, in [payment-request] note the guards used around the [[state]] internal slot, such as in the show() algorithm.

• if the deferred event doesn’t need extra state, or a state machine, this probably means that the event is just signalling the completion of the algorithm. If this is true, the API should probably return a Promise instead of firint the event. See § 7.1 Use promises for one time events.

Note: events that expose the possibility of recursion as described in this section were sometimes called "synchronous events". This terminology is discouraged as it implies that it’s possible to dispatch an event asynchronously. All events are dispatched synchronously. What is more often implied by "asynchronous event" is to defer firing an event.

7.7. Use plain Events for state

Where possible, use a plain Event with a specified type, and capture any state information in the target object.

It’s usually not necessary to create new subclasses of Event.

7.8. Use Events and Observers appropriately

In general, use EventTarget and notification Events, rather than an Observer pattern, unless an EventTarget can’t work well for your feature.

Using an EventTarget ensures your feature benefits from improvements to the shared base class, such as the addition of the once.

If using events causes problems, such as unavoidable recursion, consider using an Observer pattern instead.

MutationObserver, Intersection Observer, Resize Observers, and IndexedDB Observers are all examples of an Observer pattern.

The Observer pattern works like this:

• Each instance of the Observer class is constructed with a callback, and optionally with some options to customize what should be observed.

• Instances begin observing specific targets, using a method named observe(), which takes a reference to the target to be observed. The options to customize what should be observed may be provided here instead of to the constructor. The callback provided in the constructor is invoked when something interesting happens to those targets.

• Callbacks receive change records as arguments. These records contain the details about the interesting thing that happened. Multiple records can be delivered at once.

• The author may stop observing by calling a method called unobserve() or disconnect() on the Observer instance.

• Optionally, a method may be provided to immediately return records for all observed-but-not-yet-delivered occurrences.

function checkElementStillVisible(element, observer) {
    delete element.visibleTimeout;

    // Process any observations which may still be on the task queue
    processChanges(observer.takeRecords());

    if ('isVisible' in element) {
        delete element.isVisible;
        logAdImpressionToServer();

        // Stop observing this element
        observer.unobserve(element);
    }
}

function processChanges(changes) {
    changes.forEach(function(changeRecord) {
        var element = changeRecord.target;
        element.isVisible = isVisible(changeRecord.boundingClientRect,
                                      changeRecord.intersectionRect);
        if ('isVisible' in element) {
            // Element became visible
            element.visibleTimeout = setTimeout(() => {
                checkElementStillVisible(element, observer);
            }, 1000);
        } else {
            // Element became hidden
            if ('visibleTimeout' in element) {
                clearTimeout(element.visibleTimeout);
                delete element.visibleTimeout;
            }
        }
    });
}

// Create IntersectionObserver with callback and options
var observer = new IntersectionObserver(processChanges,
                                        { threshold: [0.5] });

// Begin observing "ad" element
var ad = document.querySelector('#ad');
observer.observe(ad);

To use the Observer pattern, you need to define:

1. the new Observer object type,

2. an object type for observation options, and

3. an object type for the records to be observed.

The trade-off for this extra work is the following advantages:

• Instances can be customized at observation time, or at creation time. The constructor for an Observer, or its observe() method, can take options allowing authors to customize what is observed for each callback. This isn’t possible with addEventListener().

• It’s easy to stop listening on multiple callbacks using the disconnect() or unobserve() method on the Observer object.

• You have the option to provide a method like takeRecords(), which immediately fetches the relevant data, instead of waiting for an event to fire.

• Because Observers are single-purpose, you don’t need to specify an event type.

Observers and EventTargets have these things in common:

• Both can be customized at creation time.

• Both can batch occurrences and deliver them at any time. EventTargets don’t need to be synchronous; they can use microtask timing, idle timing, animation-frame timing, etc. You don’t need an Observer to get special timing or batching.

• Neither EventTargets nor Observers need to participate in a DOM tree (bubbling/capture and cancellation). Most prominent EventTargets are Nodes in the DOM tree, but many other events are standalone; for example, IDBDatabase and XMLHttpRequestEventTarget. Even when using Nodes, your events may be designed to be non-bubbling and non-cancelable.

const io = new ETIntersectionObserver(element, { root, rootMargin, threshold });

function listener(e) {
    for (const change of e.changes) {
        // ...
    }
}

io.addEventListener("intersect", listener);
io.removeEventListener("intersect", listener);

See also:

• § 7.5 Use events for notification

• § 7.7 Use plain Events for state

8. Web IDL, Types, and Units

8.1. Use numeric types appropriately

If an API you’re designing uses numbers, use one of the following [WEBIDL] numeric types, unless there is a specific reason not to:

unrestricted double

Any JavaScript number, including infinities and NaN

double

Any JavaScript number, excluding infinities and NaN

[EnforceRange] long long

Any JavaScript number from -2⁶³ to 2⁶³, rounded to the nearest integer. If a number outside this range is given, the generated bindings will throw a TypeError.

[EnforceRange] unsigned long long

Any JavaScript number from 0 to 2⁶⁴, rounded to the nearest integer. If a number outside this range is given, the generated bindings will throw a TypeError.

JavaScript has only one numeric type, Number: IEEE 754 double-precision floating point, including ±0, ±Infinity, and NaN. [WEBIDL] numeric types represent rules for modifying any JavaScript number to belong to a subset with particular properties. These rules are run when a number is passed to the interface defined in IDL, whether a method or a property setter.

If you have extra rules which need to be applied to the number, you can specify those in your algorithm.

bigint should be used only when values greater than 2⁵³ or less than -2⁵³ are expected.

An API should not support both BigInt and Number simultaneously, either by supporting both types via polymorphism, or by adding separate, otherwise identical APIs which take BigInt and Number. This risks losing precision through implicit conversions, which defeats the purpose of BigInt.

8.2. Represent strings appropriately

When designing a web platform feature which operates on strings, use DOMString unless you have a specific reason not to.

Most string operations don’t need to interpret the code units inside of the string, so DOMString is the best choice. In the specific cases explained below, it might be appropriate to use either USVString or ByteString instead. [INFRA] [WEBIDL]

USVString is the Web IDL type that represents scalar value strings. For strings whose most common algorithms operate on scalar values (such as percent-encoding), or for operations which can’t handle surrogates in input (such as APIs that pass strings through to native platform APIs), USVString should be used.

Reflecting IDL attributes whose content attribute is defined to contain a URL (such as href) should use USVString. [HTML]

ByteString should only be used for representing data from protocols like HTTP which don’t distinguish between bytes and strings. It isn’t a general-purpose string type. If you need to represent a sequence of bytes, use Uint8Array.

8.3. Use milliseconds for time measurement

If you are designing an API that accepts a time measurement, express the time measurement in milliseconds.

Even if seconds (or some other time unit) are more natural in the domain of an API, sticking with milliseconds ensures that APIs are interoperable with one another. This means that authors don’t need to convert values used in one API to be used in another API, or keep track of which time unit is needed where.

This convention began with setTimeout() and the Date API, and has been used since then.

Note: high-resolution time is usually represented as fractional milliseconds using a floating point value, not as an integer value of a smaller time unit like nanoseconds.

8.4. Use the appropriate type to represent times and dates

When representing date-times on the platform, use the DOMHighResTimeStamp type. DOMHighResTimeStamp allows comparison of timestamps, regardless of the user’s time settings.

DOMHighResTimeStamp values represent a time value in milliseconds. See [HIGHRES-TIME] for more details.

Don’t use the JavaScript Date class for representing specific date-time values. Date objects are mutable (may have their value changed), and there is no way to make them immutable.

8.5. Use Error or DOMException for errors

Represent errors in web APIs as ECMAScript error objects (e.g., Error) or as DOMException. whether they are exceptions, promise rejection values, or properties.

9. OS and Device Wrapper APIs

New APIs are now being developed in the web platform for interacting with devices. For example, authors wish to be able to use the web to connect with their microphones and cameras, generic sensors (such as gyroscope and accelerometer), Bluetooth and USB-connected peripherals, automobiles, etc.

These can be functionality provided by the underlying operating system, or provided by a native third-party library to interact with a device. These are an abstraction which "wrap" the native functionality without introducing significant complexity, while securing the API surface to the browser. So, these are called wrapper APIs.

This section contains principles for consideration when designing APIs for devices.

9.1. Use care when exposing identifying information about devices

If you need to give web sites access to information about a device, use the guidelines below to decide what information to expose.

Firstly, think carefully about whether it is really necessary to expose identifying information about the device at all. Consider whether your user needs could be satisfied by a less powerful API.

Exposing the presence of a device, additional information about a device, or device identifiers, each increase the risk of harming the user’s privacy.

One risk is that as more specific information is shared, the set of fingerprinting data available to sites gets larger. There are also other potential risks to user privacy.

Privacy Threat Model is not ready for prime time.

If there is no way to design a less powerful API, use these guidelines when exposing device information:

Limit information in the id

Include as little identifiable information as possible in device ids exposed to the web plaform. Identifiable information includes branding, make and model numbers, etc You can usually use a random number or a unique id instead. Make sure that your ids aren’t guessable, and aren’t re-used.

Keep the user in control

When the user chooses to clear browsing data, make sure any stored device ids are cleared.

Hide sensitive ids behind a user permission

If you can’t create a device id in an anonymous way, limit access to it. Make sure the user can provide meaningful consent to a Web page accessing this information.

Tie ids to the same-origin model

Create distinct unique ids for the same physical device for each origin that has has access to it.

If the same device is requested more than once by the same origin, return the same id for it (unless the user has cleared their browsing data). This allows authors to avoid having several copies of the same device.

Persistable when necessary

If a device id is time consuming to obtain, make sure authors can store an id generated in one session for use in a later session. You can do this by making sure that the procedure to generate the id consistently produces the same id for the same device, for each origin.

See also:

• [LEAST-POWER]

• [FINGERPRINTING-GUIDANCE]

• [UNSANCTIONED-TRACKING]

9.2. Use care when exposing APIs for selecting or enumerating devices

Look for ways to avoid enumerating devices. If you can’t avoid it, expose the least information possible.

If an API exposes the the existence, capabilities, or identifiers of more than one device, all of the risks in § 9.1 Use care when exposing identifying information about devices are multiplied by the number of devices. For the same reasons, consider whether your user needs could be satisfied by a less powerful API. [LEAST-POWER]

If the purpose of the API is to enable the user to select a device from the set of available devices of a particular kind, you may not need to expose a list to script at all. An API which invokes a User-Agent-provided device picker could suffice. Such an API:

• keeps the user in control,

• doesn’t expose any device information without the user’s consent,

• doesn’t expose any fingerprinting data about the user’s environment by default, and

• only exposes information about one device at a time.

When designing API which allows users to select a device, it may be necessary to also expose the fact that there are devices are available to be picked. This does expose one bit of fingerprinting data about the user’s environment to websites, so it isn’t quite as safe as an API which doesn’t have such a feature.

If you must expose a list of devices, try to expose the smallest subset that satisfies your user needs.

For example, an API which allows the website to request a filtered or constrained list of devices is one option to keep the number of devices smaller. However, if authors are allowed to make multiple requests with different constraints, they may still be able to access the full list.

Finally, if you must expose the full list of devices of a particular kind, please rigorously define the order in which devices will be listed. This can reduce interoperability issues, and helps to mitigate fingerprinting. (Sort order could reveal other information: see Fingerprinting Guidance § 5.2 Standardization for more.)

Note: While APIs should not expose a full list of devices in an implementation-defined order, they may need to for web compatibility reasons.

9.3. Design based on user needs, not the underlying API or hardware

Expose new native capabilities being brought to the web based on user needs.

Avoid directly translating an existing native API to the web.

Instead, consider the functionality available from the native API, and the user needs it addresses, and design an API that meets those user needs, even if the implementation depends on the existing native API.

Be particularly careful about exposing the exact lifecycle and data structures of the underlying native APIs. When possible, consider flexibility for new hardware.

This means newly proposed APIs should be designed with careful consideration on how they are intended to be used rather than how the underlying hardware, device, or native API available today.

9.4. Be proactive about safety

When bringing native capabilities to the web platform, try to design defensively.

Bringing a native capability to the web platform comes with many implications. Users may not want websites to know that their computers have specific capabilities. Therefore, access to anything outside of the logical origin boundary should be permission gated.

For example, if a device can store state, and that state is readable at the same time by multiple origins, a set of APIs that lets you read and write that state is effectively a side-channel that undermines the origin model of the web.

For these reasons, even if the device allows non-exclusive access, you may want to consider enforcing exclusive access per-origin, or even restricting it further to only the current active tab.

Additionally, APIs should be designed so that the applications can gracefully handle physical disruption, such as a device being unplugged.

9.5. Adapt native APIs using web platform principles

When adapting native operating system APIs for the web, make sure the new web APIs are designed with web platform principles in mind.

Make sure the web API can be implemented on more than one platform

When designing a wrapper API, consider how different platforms provide its functionality.

Ideally, all implementations should work exactly the same, but in some cases you may have a reason to expose options which only work on some platforms. If this happens, be sure to explain how authors should write code which works on all platforms. See § 2.5 New features should be detectable.

Underlying protocols should be open

APIs which require exchange with external hardware or services should not depend on closed or proprietary protocols. Depending on non-open protocols undermines the open nature of the web.

Design APIs to handle the user being off-line

If an API depends on some service which is provided by a remote server, make sure that the API functions well when the user can’t access the remote server for any reason.

Avoid additional fingerprinting surfaces

Wrapper APIs can unintentionally expose the user to a wider fingerprinting surface. Please read the TAG’s finding on unsanctioned tracking for additional details.

10. Other API Design Considerations

10.1. Enable polyfills for new features

Polyfills can be hugely beneficial in helping to roll out new features to the web platform. The Technical Architecture Group finding on Polyfills and the Evolution of the Web offers guidance that should be considered in the development of new features, notably:

• Being "polyfillable" isn’t essential but is beneficial

• § 2.5 New features should be detectable

• Polyfill development should be encouraged

10.2. Where possible APIs should be made available in DedicatedWorker

When exposing a feature, please consider whether it makes sense to expose the feature to DedicatedWorker as well.

Many features could work out of the box on a DedicatedWorker and not enabling the feature there could limit the ability for users to run their code in a non blocking manner.

Certain challenges can exist when trying to enable a feature on DedicatedWorker, especially if the feature requires user input by asking for permission, or showing a picker or selector. Even though this might discourage spec authors to support workers, we still recommend designing the feature with DedicatedWorker support in mind, in order to not add assumptions that will later make it unnecessarily hard to expose these APIs to DedicatedWorker.

10.3. Add new data formats properly

Always define a corresponding MIME type and extend existing APIs to support this type for any new data format.

There are cases when a new capability on the web involves adding a new data format. This can be an image, video, audio, or any other type of data that a browser is expected to ingest. New formats should have a standardized MIME type, which is strictly validated.

While legacy media formats do not always have strict enforcement for MIME types (and sometimes rely on peeking at headers, to workaround this), this is mostly for legacy compatibility reasons and should not be expected or implemented for new formats.

It is expected that spec authors also integrate the new format to existing APIs, so that they are safelisted in both ingress (e.g. decoding from a ReadableStream) and egress (e.g. encoding to a WriteableStream) points from a browser’s perspective.

For example. if you are to add an image format to the web platform, first add a new MIME type for the format. After this, you would naturally add a decoder (and presumably an encoder) for said image format to support decoding in HTMLImageElements. On top of this, you are also expected to add support to egress points such as HTMLCanvasElement.toBlob() and HTMLCanvasElement.toDataURL().

For legacy reasons browsers support MIME type sniffing, but we do not recommend extending the pattern matching algorithm, due to security implications, and instead recommend enforcing strict MIME types for newer formats.

New MIME types should have a specification and should be registered with the Internet Assigned Numbers Authority (IANA).

10.4. Conform new HTTP Headers to standards

If you are defining a new HTTP header, its syntax mustn’t go against the HTTP specification.

If the new header must convey structured data, such as lists, dictionaries, or typed values like decimals, strings, or booleans, then the header should use the syntax defined in Structured Field Values for HTTP. This avoids consumers of the header having to write and maintain specific micro-parsers, or even worse, something that would break those existing parsers. If the new header requires data that can’t be represented by Structured Field Values, then either engage with IETF about extending the Structured Field Values syntax, or re-consider if an HTTP header is a right place to expose the data before inventing a new syntax. [RFC8941]

10.5. Extend existing manifest files rather than creating new ones

If your feature requires a manifest, investigate whether you can extend an existing manifest schema.

New web features should be self-contained and self-describing and ideally should not require an additional manifest file. Some of the existing manifest files include

• Web App Manifest which contains features related to web applications.

• Payment Method Manifest which is used for payment methods in the context of the web payment API

• Publication Manifest which is used by some web publications working group standards

• Origin Policy which is used to set security policies.

We encourage people to extend existing manifest files. Always try to get the changes into the original spec, or at least discuss the extension with the spec editors. Having this discussion is more likely to result in a better design and lead to something that better integrates with the platform.

When designing new keys and values for a manifest, make sure they are needed (that is, they enable well-thought-out use-cases). Also, please check if a similar key exists. If an existing key/value pair does more or less what is needed, work with the existing spec to extend it to your use-case if possible.

However, if your feature requires a complex set of metadata specific to a functional domain, the creation of a new manifest may be justified.

You may need to make a new manifest file if the domain of the manifest file is different from the existing manifest files. For example, if the fetch timing is different, or if the complexity of the manifest warrants it. Application metadata should be added to the Web App Manifest or be an extension of it. Manifests designated to be used for specific applications or which require interoperability with non-browsers may need to take a different approach. Payment Method Manifest, Publication Manifest, and Origin Policy are examples of these cases.

For example, if you have a single piece of metadata, even if the fetch timing is different than an existing manifest, it is probably best to use an existing manifest (or ideally design the feature in such a way that a manifest is not required). However, if your feature requires a complex set of metadata specific to a functional domain, the creation of a new manifest may be justified.

Note that in all cases, the naming conventions should be harmonized (see § 12 Naming principles).

Note: By principle, existing manifests use lowercase, underscore-delimited names. There have been times where it was useful to re-use dictionaries from a manifest in DOM APIs as well, which meant converting the names to camel-cased version. One such example is the image resource. For this reason, if a key can clearly be expressed as a single word, that is recommended.

10.6. Ensure features are developer-friendly

Any new feature should be developer-friendly. While it is hard to quantify friendliness, at least consider the following points.

While error text in exceptions should be generic, developer-oriented error messages (such as those from a developer console) must be meaningful. When a developer encounters an error, the message should be specific to that error case, and not overly generic.

Ideally, developer-oriented error messages should have enough information to guide the developer in pinpointing where the problem is.

Declarative features such as CSS, may require extra work in the implementation for debuggability. Defining this in the specification not only makes the feature more developer-friendly, it also ensures a consistent development experience for the users.

A good example where debuggability was defined as part of the specification is Web Animations.

10.7. Use the best crypto, and expect it to evolve

Use only cryptographic algorithms that have been impartially reviewed by security experts, and make sure your choice of algorithm is proven, and up-to-date. Not only do they become obsolete or insecure, cryptographic protocols and algorithms also evolve quickly.

11. Writing good specifications

This document mostly covers API design for the Web, but those who design APIs are hopefully also writing specifications for the APIs that they design.

11.1. Identify the audience of each requirement in your specification

Document both how authors should write good code using your API, and how implementers of your API should handle poorly-written code.

The web, especially in comparison to other platforms, is designed to be robust in accepting poorly-formed markup. This means that web pages which use older versions of web standards can still be viewed in newer user agents, and also that authors have a shallower learning curve.

To support this, web specification writers need to describe how to interpret poorly-formed markup, as well as well-formed markup.

Implementers need to be able to understand the "supported language", which is more complex than the "conforming language" which authors should be aiming to use.

11.2. Specify completely and avoid ambiguity

When specifying how a feature should work, make sure that there is enough information so that authors don’t have to write different code to work with different implementations.

If a specification isn’t specific enough, implementers might make different choices which force authors to write extra code to handle the differences.

Implementers shouldn’t need to check details of other implementations to avoid this situation. Instead, the specification should be complete and clear enough on its own.

Note: This doesn’t mean that implementations can’t render things differently, or show different user interfaces for things like permission prompts.

Note: Implementers should file bugs against specifications which don’t give them clear enough information to write the implementation.

11.2.1. Define algorithms clearly

Write algorithms in a way that is clear and concise.

The most common way to write algorithms is to write an explicit sequence of steps. This often looks like pseudo-code.

When writing a sequence of steps, imagine that it is a piece of functional code.

• Clearly specify the inputs and outputs, name the algorithm and the variables it uses well, and explicitly note the points in the algorithm where the algorithm may return a result or error.

• As much as possible, avoid writing algorithms which have side effects.

Summarize the purpose of the algorithm before going into detail, so that readers can decide whether to read the steps or skip over them. For example take the following steps, which ensure that there is at most one pending X callback per top-level browsing context.

A plain sequence of steps is not always the best way to write an algorithm. For example, it might make sense to define or re-use a formal syntax or grammar to avoid repetition, or define specific states to be used in a state machine. When using extra constructs like these, the earlier advice still applies.

As much as possible, describe algorithms as closely as possible to how they would be implemented. This may make the spec harder to write, but it means that implementations don’t need to figure out how to translate what’s written in the specification to how it should actually be implemented. In particular, that may mean that different implementations make different decisions that may lead to later features being feasible in one implementation but not another.

CSS selectors are read and understood from left to right, but in practice are matched from right to left in implementations. This allows the most specific term to be matched or not matched quickly, avoiding unnecessary work. The CSS selector matching algorithm is written this way, instead of a hypothetical algorithm which would more closely match how CSS selectors are often read by CSS authors.

See also:

• some useful definitions and terminology from [INFRA]

11.2.2. Use explicit flags for state

Instead of describing state with words, use explicit flags for state when writing algorithms.

Using explicit flags makes it clear whether or not the state changes in different error conditions, and makes it clear when the state described by the flags is reset.

11.3. Avoid monkey patching

An example of monkey patching would be: specification A defines an internal algorithm part of its capability, then specification B overrides or modifies said algorithm with functionality directly, not using publicly defined extension points.

As such, monkey patching (wrongly!) presupposes that underlying functionality cannot, and will not, be changed. This can lead to several problems:

• If the existing specification changes, the monkey patched text may no longer apply thus causing it to break or lead to confusion.

• If several specifications monkey patch part of another spec, the order in which the patches are applied can lead to inconsistent behavior.

• The existing specification authors may not be aware of the monkey patched text in your spec. This misses an opportunity to receive guidance on reaching consensus on a fix. Or worse, the behavior that is being patched is interrelated to so other issue or behavior, making the overall situation worse.

• An implementer doing code maintenance may read underlying specification and inadvertently revert the monkey patched behavior assuming it is a bug.

Wikipedia also describes some additional pitfalls of monkey patching.

11.3.1. If you need to monkey patch

1. File a bug, or create a fix, for the existing specification. This will help you understand what the problem is and how to fix it, and maybe even alleviate you from even creating the monkey patch in the first place.

2. In your spec, clearly mark the monkey patch as a temporary workaround (e.g., using a big red box) and make it clear to readers that you intend to add your patch to the existing specification. Make sure you link to the existing specification and the bug or fix that you filed.

3. Work together with the authors of the existing specification on a solution. If the existing specification is no longer maintained... Congratulations! you are now the editor of that existing specification. If necessary, add suitable extension points to the existing specification to better handle your changes.

4. Once the existing specification has integrated your monkey patch, remove the monkey patch from your specification.

Note that monkey patching is different from "modularization", which extends existing technology in a way that is self-contained and doesn’t cause side-effects to other underlying specifications (e.g., the CSS Modules). Modularization is considered a good practice.

It’s also usually ok to extend a specification using WebIDL’s partial interfaces, partial dictionaries, and so on. But even in those cases, it’s highly advisable to coordinate with the authors of the specification being extended.

12. Naming principles

Names take meaning from:

• signposting (the name itself)

• use (how people come to understand the name over time)

• context (the object on the left-hand side, for example)

12.1. Use common words

API naming must be done in easily readable US English. Keep in mind that most web developers aren’t native English speakers. Whenever possible, names should be chosen that use common vocabulary a majority of English speakers are likely to understand when first encountering the name.

Value readability over brevity. Keep in mind, however, that sometimes the shorter name is the clearer one. For instance, it may be appropriate to use technical language or well-known terms of art in the specification where the API is defined.

12.2. Use ASCII names

Names must adhere to the local language restrictions, for example CSS ident rules etc. and should be in the ASCII range.

12.3. Consult others on naming

Consult widely on names in your APIs.

You may find good names or inspiration in surprising places.

• What are similar APIs named on other platforms, or in popular libraries in various programming languages?

• Ask end users and developers what they call things that your API works with or manipulates.

• Look at other web platform specifications, and seek advice from others working in related areas of the platform.

• Also, consult if the names used are inclusive.

Pay particular attention to advice you receive with clearly-stated rationale based on underlying principles.

Use Web consistent names

Use Inclusive Language

Use inclusive language whenever possible.

For example, you should use blocklist and allowlist instead of blacklist and whitelist, and source and replica instead of master and slave.

If you need to refer to a generic persona, such as an author or user, use the generic pronoun "they", "their", etc. For example, "A user may wish to adjust their preferences".

12.4. Use future-proof names

Naming should be generic and future-proof whenever possible.

The name should not be directly associated with a brand or specific revision of the underlying technology whenever possible; technology becomes obsolete, and removing APIs from the web is difficult.

12.5. Name things consistently

Sets of related names should agree with each other in:

• part of speech - noun, verb, etc.

• negation, for example all of the names in a set should either describe what is allowed or they should all describe what is denied

Boolean properties vs. boolean-returning methods

Boolean properties, options, or API arguments which are asking a question about their argument should not be prefixed with is, while methods that serve the same purpose, given that it has no side effects, should be prefixed with is to be consistent with the rest of the platform.

Use casing rules consistent with existing APIs

Although they haven’t always been uniformly followed, through the history of web platform API design, the following rules have emerged:

12.6. Warn about dangerous features

Where possible, mark features that weaken the guarantees provided to developers by making their names start with "unsafe" so that this is more noticeable.

For example, Content Security Policy (CSP) provides protection against certain types of content injection vulnerabilities. CSP also provides features that weaken this guarantee, such as the unsafe-inline keyword, which reduces CSP’s own protections by allowing inline scripts.

13. Other resources

Some useful advice on how to write specifications is available elsewhere:

• Writing specifications: Kinds of statements (Ian Hickson, 2006)

• QA Framework: Specification Guidelines (W3C QA Working Group, 2005)

• Privacy Considerations for Web Protocols

• Self-Review Questionnaire: Security and Privacy

• Web Technology Accessibility Guidelines

• Internationalization Best Practices for Spec Developers

Acknowledgments

This document consists of principles which have been collected by TAG members past and present during TAG design reviews. We are indebted to everyone who has requested a design review from us.

The TAG would like to thank Adrian Hope-Bailie, Alan Stearns, Aleksandar Totic, Alex Russell, Andreas Stöckel, Andrew Betts, Anne van Kesteren, Benjamin C. Wiley Sittler, Boris Zbarsky, Brian Kardell, Charles McCathieNevile, Chris Wilson, Dan Connolly, Daniel Ehrenberg, Daniel Murphy, Domenic Denicola, Eiji Kitamura, Eric Shepherd, Ethan Resnick, fantasai, François Daoust, Henri Sivonen, HE Shi-Jun, Ian Hickson, Irene Knapp, Jake Archibald, Jeffrey Yasskin, Jeremy Roman, Jirka Kosek, Kevin Marks, Lachlan Hunt, Léonie Watson, L. Le Meur, Lukasz Olejnik, Maciej Stachowiak, Marcos Cáceres, Mark Nottingham, Martin Thomson, Matt Giuca, Matt Wolenetz, Michael[tm] Smith, Mike West, Nick Doty, Nigel Megitt, Nik Thierry, Ojan Vafai, Olli Pettay, Pete Snyder, Philip Jägenstedt, Philip Taylor, Reilly Grant, Richard Ishida, Rick Byers, Ryan Sleevi, Sergey Konstantinov, Stefan Zager, Stephen Stewart, Steven Faulkner, Surma, Tab Atkins-Bittner, Tantek Çelik, Tobie Langel, Travis Leithead, and Yoav Weiss for their contributions to this & the HTML Design Principles document which preceded it.

Special thanks to Anne van Kesteren and Maciej Stachowiak, who edited the HTML Design Principles document.

If you contributed to this document but your name is not listed above, please let the editors know so they can correct this omission.