Stylo (a.k.a. Quantum CSS) will integrate Servo's CSS style system into Gecko, such that the style system code can be shared by Gecko and Servo. Stylo V1 will support Firefox on Windows, macOS, and Linux. Android support may ship in a later release. Stylo is a core part of Project Quantum.

To help test Stylo, download Firefox Nightly. To confirm that Stylo is enabled, check about:support for "Stylo: true (enabled by default)". If it's not, then set the about:config pref layout.css.servo.enabled to true.

For a deep dive into Stylo internals, check out Lin Clark's Mozilla Hacks blog post "Inside a super fast CSS engine: Quantum CSS (aka Stylo)".

Getting Involved

If you're interested in contributing to Stylo or Servo, drop by the #servo IRC channel on Mozilla's IRC server.

One front where you can help out is adding new properties to Servo's CSS parsing crate. This requires familiarity with Rust, but is otherwise a good way to dip your toes into Servo's style system. To get started, see Servo's CSS property hacking guide for more information.

If you do not know Rust, you can help implement keyword properties. See the Stylo hacking guide for more details. The Stylo hacking guide also has instructions for building Stylo if you wish to play with it yourself. If you know C++, Firefox bug 1277133 has information about easy Gecko bugs that will assist Stylo's integration with Gecko.

Some Stylo bugs that are good for new Servo contributors are tagged on GitHub as “easy” bugs and “less-easy” bugs.

Committing stylo changes

The process for committing a Stylo change that involves both Gecko and Servo changes is as follows:

Create a Github PR against https://github.com/servo/servo with the Servo changes in question.
Get that reviewed and checked into Servo's github repository.
Wait for servo-vcs-sync to land the Servo changes on Gecko's autoland.
Land the Gecko changes on autoland.

A more exhaustive / overwhelming description of the workflow is also available.

Try Syntax

For most Stylo changes, it should be sufficient to build on all platforms and test on Linux 64-bit only, which helps save resources in automation. To do this, you can use:

./mach try -b do -p all -u all[x64]

To build and test all Stylo platforms running in automation, you can use:

./mach try -b do -p linux,linux64,linux64-haz,macosx64,win32,win64 -u all

To build all possible platforms to ensure code compiles everywhere, but only test with Stylo enabled, you can use:

./mach try -b do -p all -u all[linux32,linux64,linux64-stylo-sequential,macosx64,windows7-32,windows10-64]

Release Criteria

Stylo should not cause us to regress any CSS feature unless we explicitly decide that regressing that specific feature is acceptable. The existing test suite will be the primary benchmark of this, with cross-validation from an automated analysis and manual QA of the Alexa Top 500 sites.

Stylo should be enabled by default on all desktop platforms.
Stylo should have zero P1 and P2 bugs.
Stylo should pass all (non-disabled) Gecko reftests and mochitests.
Stylo should support at least as many CSS properties as than Gecko. Firefox = 375. Stylo = 375. Chrome = 276. DONE!
Stylo should pass visual diff testing for the Alexa top sites. (bug 1331552) DONE!
Stylo's sequential mode performance on Talos tp5 and tp6 should be at least as fast as Gecko's.
Stylo's parallel mode performance on Talos tp5 and tp6 should be faster than Gecko's. Ideally, Stylo should scale linearly with the number of cores.
Stylo's performance on Speedometer should be at least as fast as Gecko's.
Stylo's memory usage (as measured by AWSY and user telemetry) should not exceed to Gecko's by more than X% (TBD).
Stylo's crash rate should be less than or equal to Gecko's.
Stylo's user engagement metrics should be greater than or equal to Gecko's.

	Linux32	Linux64	macOS	Win32	Win64	Android
Build	bug 1366050	bug 1374824	bug 1375774	bug 1384258	bug 1384258	bug 1366049 (enabled 2017-11-23)
AWSY	not tested	1% regression	1% regression	2% regression	3% regression	0% change
Speedometer	not tested	linux64 Perfherder	macOS Perfherder	win7-32 Perfherder win8-32 desktop win10-32 reference laptop	win10-64 Perfherder win8-64 desktop win10-64 reference laptop	not tested
StyleBench	not tested	linux64 Perfherder	macOS Perfherder	win7-32 Perfherder	win10-64 Perfherder	not tested
tp5	not tested	linux64	macOS	win7-32	win10-64	tp4m (tiny improvement)
tp6a	not tested	not tested	macOS	win7-32	win10-64	not tested
tp6f	not tested	not tested	macOS	win7-32	win10-64	not tested
tp6g	not tested	not tested	macOS	win7-32	win10-64	not tested
tp6y	not tested	not tested	macOS	win7-32	win10-64	not tested
Autophone	not tested	not tested	not tested	not tested	not tested	remote-blank (tiny improvement) remote-nytimes (tiny regression) remote-tsvg (tiny regression bug 1420369) remote-twitter (tiny regression bug 1420369)

Milestones

stylo-central

Run linux64-stylo tests in mozilla-central (bug 1330414) (dependencies)
- Enable stop-gap vcs sync (bug 1317525)
- Vendor servo in mozilla-central (bug 1322769)
- Make all tests green (by skipping or expecting failure)

stylo-nightly

Enable Stylo in Nightly (bug 1330412) (dependencies)
- Implement parsing of all CSS properties supported by Firefox
- Fix and enable all tests (without skipping or expecting failure)
- Fix all crashes and assertion failures
  - Fuzz Stylo
  - Pass [Auto-tools/Projects/BugHunter Bughunter]
- Manual QA sign-off
  - Smoke test Alexa Top N sites
  - Exploratory testing of browser UI and features

stylo-release

Stylo meta bug 1243581 (dependencies)
Run experiment comparing Stylo vs Gecko on engagement ratio, performance, crash rate, and memory usage.
Let Stylo ride the trains
- Enable Stylo on all platforms (bug stylo-nightly)
  - Pass QA on all platforms (tests, fuzzing, BugHunter)
Ship Stylo to Release

Bugs