• Beta Chromium blog posts: For each new Chrome Beta release (~every six weeks), the Chrome team publishes a blog post outlining changes to the web platform and the Chrome Apps & Extensions APIs. Here's an example of one of these posts.
• chromestatus.com: A dashboard where we track new feature development. Learn more in the “Feature Dashboard” section below.
• bit.ly/blinkintents: A Google Spreadsheet that lists all "Intent" threads and their approval status.
• Chrome Developer Relations: Chrome DevRel posts about new features on both Twitter (@ChromiumDev) and Google+ (+GoogleChromeDevelopers).
• Blink launch tracking bugs: These bugs track new features on the Chromium issue tracker. You can find launch bugs by applying the filter Type=Launch-OWP (OWP stands for "open web platform").
• blink-reviews@chromium.org: a high-traffic list for Blink code reviews. The code for all new features gets reviewed on this list.

Committing and reviewing code

The process for becoming a committer or an OWNER is the same as for the larger Chromium project, and code changes are approved by OWNERS. Experience with the codebase from working on WebKit will be taken into account to accelerate the process of becoming an OWNER.

Developing Blink

Blink is implemented on top of an abstract platform and thus cannot be run by itself. The Chromium Content module provides the implementation of this abstract platform required for running Blink. Developing for the Content module is covered in the Chromium developer docs. Testing your changes can be done with a build of Content Shell or a full Chromium build. For some tips and suggestions on debugging Blink, see the page on Getting Started with Blink Debugging.

Blink code uses the Blink Coding Style Guidelines..

How do I port Blink to my platform?

Blink runs on an abstract platform inside a sandbox and therefore has few operating-system-specific dependencies. This design has two consequences: (1) Blink cannot run alone, and (2) porting to other platforms happens at a different layer. Instead of adding platform-specific code to Blink, you should use Chromium’s content layer, which provides an implementation of this virtual platform on a wide variety of operating systems including Windows, Linux, Mac OS X, and Android. A separate project called the Chromium Embedded Framework is probably the easiest way to use Chromium (and thus Blink) on your platform.

Web Platform Changes: Guidelines

In addition to making the web platform faster and more secure, improving the web platform also means adding new functionality and removing cruft. To fulfill our good citizenship mission, we need to be careful to change the web-facing API in a transparent, responsible, and compatible manner. We measure success as moving the open web platform forward as a whole, not just moving one implementation forward.

In practice, we strive to ensure that the features we ship by default have open standards. As we work on features, we track their progress in the web standards community with the Chromium Features Dashboard, which lets us be transparent about the status of each feature and about how we make decisions about which features to enable by default for the open web.

Scope

These guidelines cover non-trivial changes that are exposed to the open web or in some other way have implications for web developers, including changes that add or remove functionality or APIs. Changes that are not exposed to the open web (e.g., user interface features, extension APIs, or testing infrastructure) are outside the scope of this policy.

Policy for shipping and removing web platform API features

As browser developers, we find that there’s often tension between moving the web forward and preserving compatibility. On one hand, the web platform API surface must evolve to stay relevant. On the other hand, the web’s primary strength is its reach, which is largely a function of interoperability. By definition, when any vendor ships a new feature (or is the first to remove an obsolete feature), the API change is not yet interoperable, so cross-browser compatibility is sacrificed, even if briefly. This section outlines our approach to resolving this tension.

Evaluating “interoperability and compatibility risk”

Interoperability risk is the risk that browsers will not eventually converge on an interoperable implementation of the API. Interoperability at a given time point is not a well-formed concept, because unless every browser shipped new features at exactly the same time then there is a period of non-interoperability. In practice, we forecast “interoperability risk” (a) by observing the public behavior of others in the web ecosystem. Heuristics that decrease risk a priori for an unshipped feature include, in rough order of importance:

• Other vendors already shipping interoperable implementations
• A mature specification in the relevant standards body
• A shared test suite for that specification
• Positive signals from other browser vendors
• Lack of negative signals from other browser vendors
• A small API footprint

Compatibility risk is the likelihood that a change will break existing web content loaded in Chromium.  Compatibility risk is especially common with API removal, but is also a factor when adding new features (e.g. Unprefixed CSS Animations). We rely on a set of core principles and tools to evaluate and weigh compatibility risk, though given the complexity and nuance individual engineers are not expected to be familiar with the details.

Evaluating “moving the web forward”

A change to Blink’s API surface is said to “move the web forward” if it either (a) enables developers to build compelling applications for a large number of users, both now and in the future, and/or (b) reduces code complexity in Blink or Chromium.

Bucket (a) is subjective and changes over time. In 2014, we believe the most important changes for developers---and, by extension, users---are those that make the web more performant and easier to use on mobile devices. For more information, see the 2014 Blink goals brainstorming thread.

Bucket (b) is important to make sure that the web’s progress is not overly constrained by its legacy. Simplifying the codebase allows us to make performance improvements, more quickly add more important features, and reduce binary size. In most cases, we believe these benefits will extend to other browsers as well.

Balancing “moving the web forward” and “interoperability and compatibility risk”

To manage the project-wide tension between moving the web forward and interoperability and compatibility, we ask of each proposed API change: does this individual change strike the right balance between making the web better and minimizing interoperability and compatibility risk?

In an ideal world, all changes would both drastically move the web forward and involve zero interoperability and compatibility risk. In practice, this is rarely the case; features fit somewhere else on this chart:

Blink’s willingness to accept a change is largely determined by the change’s location on the chart above:

• If a change has low interoperability and compatibility risk and significantly moves the web forward, Blink usually welcomes it (e.g., shipping unprefixed CSS Transforms).
• If a change has low interoperability and compatibility risk but isn’t expected to significantly move the web forward, Blink usually still welcomes it. Occasionally, Blink will reject changes in this bucket to avoid technical complexity (e.g., not shipping our old implementation of CSS Variables).
• If a change has high interoperability risk and isn’t expected to significantly move the web forward, Blink will usually not welcome it (e.g., not shipping canvas supportsContext).
• If a change has high interoperability risk but is expected to significantly move the web forward, Blink will sometimes welcome it after careful, publicly-explained consideration (e.g. shipping Shadow DOM). In such cases, the implementer is expected to:
• Propose an editor’s draft (or equivalent) to the relevant standards group.
• Discuss the feature publicly with implementers of other browser engines.
• Take on an active commitment to shepherd the feature through the standards process, accepting the burden of possible API changes.

Of course, equal minds can disagree about the fuzzier aspects of these policies. For example, if a change increases runtime performance but consumes more battery, does it “move the web forward”? Or: what combination of the heuristics above precisely constitutes “high” compatibility risk?

To equitably resolve these kinds of questions on a per-change basis, Blink has a well-defined process for discussing and approving web-facing API changes. It consists of code reviews, announcements to blink-dev, and formal API review meetings. The most formal requirement of the process is that API changes must receive an LGTM from at least three different API owners before enabled by default on trunk. The API owners are responsible for making sure the policy described above is applied appropriately on a per-change basis.

Trivial Changes

Trivial platform changes do not need to meet the requirements above. For example, changes to existing APIs to improve compliance with web standards or to fix bugs are welcome. When in doubt, please send at a PSA to blink-dev or ask an API OWNER for advice.

However, any new API (no matter how small) is considered non-trivial. Trivial changes should be small fixes that have low risk of disrupting web developers. Trivial changes should be labeled as such in their code review, and may still require the LGTM of one API OWNER if they change the stable-build output of the webexposed tests.

If we’re unsure about the extent to which a change will impact web developers, we may ask the contributor proposing the change to provide data quantifying the impact. If a project member questions whether a change is trivial, we will err on the side of caution and ask the contributor to meet the requirements above.

Vendor Prefixes

Historically, browsers have relied on vendor prefixes (e.g., -webkit-feature) to ship experimental features to web developers. This approach can be harmful to compatibility because web content comes to rely upon these vendor-prefixed names. Going forward, instead of enabling a feature by default with a vendor prefix, we will instead keep the (unprefixed) feature behind the “enable experimental web platform features” flag in about:flags until the feature is ready to be enabled by default or exposed for a limited duration origin trial.

Web Platform Changes: Process

Launch process: launching a new feature

Launch Process: removing a feature

API Owners

Here's the current list of API OWNERS.

All members of the project are responsible for enforcing that new features follow the project’s policies. Project members who feel that a feature is violating the policy should raise the issue first with the contributor and, if that doesn’t resolve the issue, with the project’s public mailing list.

To complement this project-wide responsibility, we have a set of API owners who are listed in the OWNERS file for the expected output of tests that monitor much of the web-exposed surface area of blink (eg. these). When reviewing changes to these files, the API owners should ensure that the changes meet the project’s guidelines for new and removed features.

API Review

API Review meetings will be scheduled when API discussion over email is insufficient (per the Launch Process). API owners and contributors of features under discussion are welcome to attend. The purpose of the API Review meeting is to provide a high-bandwidth forum for discussion between API owners and feature implementers. The group makes decisions by consensus; at least three project OWNERS must be present for quorum. After the meeting the organizer will send notes, including any decisions, to blink-dev.

Feature Dashboard

To improve transparency, we track development of new features on our Feature Dashboard. For each feature, the dashboard tracks our implementation status, the feature's progress through the standards process, our understanding of the opinion of other browser vendors and other key metrics.

We associate each value with a shade of red or green, corresponding to how the value reflects our web citizenship. For example, “opposition from another browser vendor” is red and “a similar implementation in another browser” is green. Viewed in aggregate, these colors provide a quick snapshot of the project’s overall web citizenship.

The dashboard data itself is also a useful high-level record of when features were implemented and a peek at what’s coming next. If you’d like to monitor lower-level changes as they happen, check out our Gitiles and SVN logs.

Guiding Principles for Process

Testing

Openness and interoperability are core to the web platform’s philosophy and success. We are committed to delivering features that are interoperable. In addition to the checks and balances in our release process, we prioritize testing as a way to promote compatibility across browsers. It is our intention to increase our investment in testing over time.

In 2012, we submitted a comprehensive Shadow DOM conformance test suite to the W3C. We’ll continue to create conformance test suites like these, so that implementations for new features are interoperable from the beginning. To incentivize this, the feature dashboard has a column for conformance tests. If a feature has a test suite, its cell in that column is green. Otherwise, it’s red.

Going forward, we’ll be working with the W3C and the broader web community to share more tests and testing infrastructure as a way to encourage interoperability.

We’ve also been collaborating with Adobe to host Test the Web Forward events, where web developers and spec authors work together to write conformance tests that all browsers are evaluated against. So far (April 2013) there have been four such events, the most recent hosted by Google in Sydney. We're committed to this effort and are currently organizing the next event in Tokyo.

If you encounter a bug in Blink or a browser interoperability issue, please use the "New issue" wizard at crbug.com.

Architectural Changes

With Blink we’re excited about the freedom to dream big for the Web. When Chromium started, our goal was to change as little of WebKit as possible, easing integration with the WebKit codebase. With Blink we are excited to make large-scale architectural changes to the code, without having to worry about breaking other consumers of WebKit.

One change we’re planning is adding “out-of-process iframes”. These allow Chromium to separate individual parts of a page into separate sandboxed processes. Implementing this will require large restructuring of how iframes are handled in WebKit. Some of this restructuring is incompatible with other WebKit ports and has thus been delayed until now.

As another example, we’d like to fix our networking code to be faster and simpler. Our current networking code in WebKit is limited by old Mac WebKit API obligations which cannot be changed. Chromium has worked around some of these limitations over the years, but these workarounds have proven fragile and have long been a source of bugs. With Blink, we’re excited to refresh this networking code without forcing other WebKit consumers to break their WebKit API obligations.

Finally, we’d like to explore even larger ideas like moving the entire Document Object Model (DOM) into JavaScript. This has the potential to make JavaScript DOM access dramatically faster, but will involve a very large re-write of WebKit’s DOM implementation—something that would be difficult in WebKit which has two supported JavaScript engines.

Some of the other changes we're considering:

• Teach WebCore about multi-process history (currently it assumes same-process synchronous History access)
• Delete the Widget tree (a Mac WebKit1 constraint)
• Split WebCore into modules
• Experiment with moving the DOM into the JS heap
• Increase multicore use (e.g., html parser, style engine, javascript parser)
• Remove obscure parts of the DOM and make backwards-incompatible changes to obscure parts of the DOM that benefit performance or remove complexity.
• Use a modern, faster tcmalloc throughout all of Mac chrome
• Experiment with incremental or parallel layout
• Fix memory leaks by removing the ScriptValue/ScriptState abstractions now that there’s only one JavaScript engine.
• Remove custom JavaScript bindings code
• Bring WebCore up to speed with DOM3 Events / [DOM] UI Events.

Sub Teams

• Animation Team
• Binding Team
• DOM Team, including HTML, XML and editing.
• Input Team
• Layout Team
• Memory Team
• Paint Team
• Web Platform Predictability
• Style Team
• Worker Team

Developer FAQ

You can find answers to some of the most common web developer-facing questions in the Blink Developer FAQ.

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