Blink is the rendering engine used by Chromium.
*To improve the open web through *
Blink is an inclusive open-source community that values fostering a supportive culture. We welcome participation from anyone who shares our mission.
Transparency is one of the core values of Blink. Our goal is for anyone to be able to participate, regardless of organizational affiliation. There are a number of areas where discussions take place:
For web developers interested in tracking new features, the signal-to-noise ratio of discussion forums might be too low to be useful. There are several dedicated channels for staying up-to-date on new features:
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.
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..
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.
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.
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.
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:
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:
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 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.
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.
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 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@.
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.
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.
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:
Changes previously considered and now done:
Platform layering
This document lays out the preferred code architecture for what classes and directories can refer to each other. The Blink Onion Soup project is currently working to organize code according to that architecture (it's a work in progress). New code should conform to that architecture.
The larger Blink team is divided in to a set of sub-teams each specializing in a particular part of the problem space and code base.
You can find answers to some of the most common web developer-facing questions in the Blink Developer FAQ.
