Hello everyone,

I would like to start a discussion on a policy to enforce WPT usage as a first 
choice, that would be enforced via check-webkit-style on Changelog files.

Why use WPT?

Contributing to WPT has many benefits:
interoperability/compatibility issues with WPT we write may be detected by 
other browser vendors and we would get faster feedback and turnaround to fix 
creates/encourages discussion in case of disagreement with other browser vendors
WebKit greatly benefits from WPT coverage, it is time to provide our own 
coverage to other browsers
Improves WebKit’s score generally for WPT (which has tests mostly contributed 
by Chromium, Firefox at the moment)

Are there reasons to not use WPT?

Common reasons for not writing WPT that have been mentioned are:
"WPTs are less pleasant to write.”
This is not true imo, the WPT harness is documented 
<https://web-platform-tests.org/writing-tests/index.html>), unlike WebKit 
internals, making it easier for new contributors to figure out things.

“When you actually move a regression test to WPT proper, commit history is 
lost, and you don't know what kind of user facing problem it's preventing any 
Use `<link rel=“help” href=“…”>` with a reference to the webkit bug. Ensure you 
actually export the WPT and merge the WPT PR as well!

There are things WPT can’t test
True (e.g. tree dumps). My proposal below covers that.

Here is my current proposal:

Every LayoutTests/ changelog adding new test files would contain:

WPT-exemption-reason:  NONE (OOPS!)

If that line is removed or no valid reason is provided, webkit-style would 
throw out an error. Valid reasons could be:

WPT-exemption-reason: urgency (webkit.org/b/XXX <http://webkit.org/b/XXX>)
I’m personally not in favour of this reason existing, but some have expressed 
interest in it, this may be used in case of urgency (security fixes etc.), 
where a bug must be filed to port it to WPT (alternatively the test path could 
be added to a new allowlist file). Eventually this reason would be removed if 
two way sync with the WPT repo is available.

WPT-exemption-reason: test-harness-deficiency (webkit.org/b/XXX 
If the WebKit implementation of the WPT harness lacks a certain feature that 
should be implemented in the WPT harness, a new bug may be filed, or an 
existing bug may be referenced.
Maintainers of the WPT harness may watch this reason to prioritize work needed 
on the harness.

WPT-exemption-reason: requires-internals: NONE (OOPS!)
requires-internals should have a valid explanation of which internals are 
needed. (Tree dumps would be a valid reason, although usually reftests can 
cover those)

In addition: 

WPT-PR: https://github.com/web-platform-tests/wpt/pulls/XX 
<https://github.com/web-platform-tests/wpt/pulls/XX> must be present in 
LayoutTests/imported/w3c/Changelog, this is to ensure tests are not lost at the 
next import.

This would also be enforced in webkit-style, although eventually this might be 
replaced by a fully automated process in the future similar to Chrome/Firefox.

Any feedback is absolutely welcome!

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