ExposureGuidelines

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Guiding Principles

Mozilla aims to advance the state of the web platform with new features. In past years, Mozilla has shipped experimental features with a "moz" prefix to indicate their lack of standardization (e.g., mozRequestAnimationFrame()). Unfortunately, this approach turned out to be harmful to the web as experimental features ended up being used in some websites before they were ready. In many cases, this meant that we were unable to innovate on certain features because to change them would break content on the web. Browsers have in some cases also been forced to implement each other's prefixed features. Therefore, to allow us to continue innovating without negatively affecting content on the web, Mozilla will no longer ship new "moz"-prefixed features (see Henri Sivonen's proposal).

We will instead make experimental features available behind preferences which can be toggled through about:config. Once we feel there is an acceptable level of consensus in the web community about the stability of an feature and we feel it is ready, we will make it generally available to the web platform (more details below on this process). We feel this strikes the right balance between allowing developers to experiment with new features, while at the same time protecting the Web from being exposed to new functionality prematurely.

Special Cases

There may be special cases where we deviate from this goal. New user-facing products may need to ship features that have not yet been embraced by other browser engines or thoroughly discussed by standards bodies. This allows Mozilla to provide functionality that other browser engines aren't working on or the majority of the Web community isn't interested in at that time. We will avoid exposing such functionality to the web at large. Developing these features without much involvement from the Web community comes with the price of temporary proprietary features. This clearly indicates their lack of standardization at that time and limits the number of developers relying upon them. Mozilla will learn from such efforts and use this knowledge to inform various standardization efforts. Our aim is to standardize our proprietary features as soon as possible so they become available on a royalty-free basis for the benefit of the web community at large.

Guidelines for Mozillians developing new features

The following is put forward as a set of guidelines for Mozillians -- especially module owners and peers -- to refer to while developing new APIs for the Web. The primary question that must be asked is:

Is this good for the web?

If the answer to that question is yes, look below for some guidance on how you should proceed with shipping your new feature. Note that trivial changes need not worry about these suggested guidelines.

When is an feature ready to be shipped by Gecko?

Features which Mozilla makes available by default to the web on the release channel should be "ready". This can mean that they are de jure standards (e.g., a W3C Recommendation) or de facto standards. Indications that a feature is ready for shipping to the web include:

  1. other browser engines ship compatible implementations in their releases or behind a preference with clear signals it will graduate to being on by default
  2. other browser engines state their intention to ship a compatible implementation
  3. a large number of web developers indicate their satisfaction with the feature design
  4. there exists a specification that is no longer at risk of significant changes, is on track to become a standard with a relevant standards body, and is acceptable to a number of applicable parties and other browser engines

How do we know what other browser engines think?

  • some of them participate in dev-platform discussions
  • watch for positive signals on the specification's discussion forum (likely a GitHub repository)
  • watch for "intent to implement" emails on mailing lists such as blink-dev
  • lack of feedback will not delay our implementation as it may simply indicate lack of interest at that time from another browser engine

How do we know what web developers want?

How do we let other browser engines know what we think?

  • participate in public discussions of proposed features
  • blog and/or tweet about proposed features
  • comment on "intent to implement" threads on blink-dev

Suggested implementation process

If your work is going to expose a new feature to the web or non-trivially change an existing one, it is requested that you follow these steps:

  1. Email dev-platform declaring your intent to implement. If you are implementing a feature that is working its way through a standards body process such as the W3C's, it's best to email as soon as possible (i.e., before W3C CR status if possible).
  2. Implement as normal. Code review rounds will take place, etc. Module owners or their designated peers will provide a super-review for all interface changes.
  3. Restrict the feature to secure contexts.
  4. Ensure the feature has web-platform-tests coverage.
  5. Land potentially disruptive features behind a preference which is off by default.

Turning a preference on by default

When you are satisfied that your new feature is stable enough for exposure to the web at large (see above), it is requested that implementers send another email to dev-platform declaring their intent to ship. Among other things, this email serves to inform those that are interested in the new API but have not been closely following the implementation bug(s).

Guidelines for removing features

Occasionally there is a need to remove features. These could be features that are proprietary to Mozilla, were standardized but never got adopted by all browsers, or features that are so bad that all browsers want to get rid of them together.

In order to make this possible while impacting users as little as possible the following steps are to be taken:

  • Gather telemetry on the feature.
  • Consult dev-platform with an "Intent to unship" that includes the telemetry data.
  • Indicate in the developer console whenever the feature is used that it's deprecated and might be removed. It's best to avoid doing this until there's agreement that the feature can be removed (which requires telemetry) as otherwise developers are needlessly spammed in the console.
  • Remove the feature when the telemetry data and dev-platform agreement indicate it can be.

Decision making

If the dev-platform thread results in a conflict, the respective module owner is responsible for resolving that conflict and making a decision on how to proceed.

Email templates

Intent to implement

To: dev-platform@lists.mozilla.org
Subject: Intent to implement: <your feature goes here>

Summary: elevator pitch for the new functionality including benefits to web developers
Bug: link to main implementation tracking bug
Link to standard: if no formal specification exists yet, link to public discussions with other browser vendors
Platform coverage: where will this be available? Android, Desktop, only exposed to privileged apps (certified app-only functionality does not require an email), etc.
Estimated or target release: in which version do you want to/plan to release this?
Preference behind which this will be implemented: if applicable, how can interested parties test this before it ships pref'd on by default?
Is this feature enabled by default in sandboxed iframes? If not, is there a proposed sandbox flag to enable it? If allowed, does it preserve the current invariants in terms of what sandboxed iframes can do?
DevTools bug: link to a Firefox :: Developer Tools bug coordinating work with the DevTools team to build tools for this feature.
Do other browser engines implement this? Answer with: shipped (since version X, behind what flags if any); intent to implement emailed (mailing list URL); considering (citation).
web-platform-tests: Please link to the test suite. If any part of the feature is not tested by web-platform-tests, please include links to one or more of:

  • A web-platform-tests issue explaining why a certain thing cannot be tested (example).
  • A spec issue for some change that would make it possible to test (example).
  • A Bugzilla bug for the creating web-platform-tests.

Is this feature restricted to secure contexts? Provide justification if this feature is not restricted to secure contexts. Otherwise, use "Yes".


Suggested additions

The above is the minimum required that should be in an "Intent to implement" email. If you've covered those, you're good, and brevity is a virtue.

If you're looking for extra credit, or to preempt common questions, consider adding any or all of the following (all based on existing dev-platform examples, and questions asked on dev-platform in response to intent to ship emails).

  • How stable is the spec: Note that even if it's unstable that shouldn't stop us implementing; that mostly affects shipping. So as long as we're pretty sure that the basic set of functionality is stable, even if the actual names of the values are not, implementing makes sense.
  • Security & Privacy Concerns: consider providing a link to answers in this security/privacy questionnaire for a spec feature, if the spec doesn't already answer it. In particular, consider if the spec exposes new information about a user's computer or behavior that can contribute to fingerprinting.
  • Web designer / developer use-cases AKA Why a developer would use Feature X?: Provide a URL to at least briefly documented use-cases for web designers and developers that illustrate why and when they would use this feature.
  • Example: Provide a brief code sample on how to use the API. Even with a formal specification, not everyone will know about the feature just from the name of the spec. An example will make it easier to understand how this feature can be used. This can either be an inline code sample, or a direct link to an example on the web.

Intent to ship

To: dev-platform@lists.mozilla.org
Subject: Intent to ship: <your feature goes here>

As of <target date> I intend to turn <feature> on by default [<on these platforms>]. It has been developed behind the <pref> preference. Other UAs shipping this or intending to ship it are <details>.

Bug to turn on by default: link to main relevant bug (https://bugzilla.mozilla.org/show_bug.cgi?id=) [note: this bug should ideally have the dev-doc-needed keyword set]

This feature was previously discussed in this "intent to implement" thread: <https://groups.google.com/forum/#!forum/mozilla.dev.platform>. If anything changed since that thread please include that information in this email.

Intent to implement and ship

To: dev-platform@lists.mozilla.org
Subject: Intent to implement and ship: <your feature goes here>

It's acceptable to combine the "intent to implement" and "intent to ship" emails as long as all the relevant requirements are met.


Intent to unship

To: dev-platform@lists.mozilla.org
Subject: Intent to unship: <your feature goes here>

<The body of the email should contain rationale, telemetry analysis, links to related discussions if any, and developer console suggestions.>

See Also