CSS Color 5: Relative Color Syntax

[svgeesus]

Request for Mozilla Position on an Emerging Web Specification

• Specification Title: CSS Color Module Level 5, section 4. Relative Color Syntax
• Specification or proposal URL (if available): https://drafts.csswg.org/css-color-5/#relative-colors
• Explainer URL (if available): RCS Explainer
• Caniuse.com URL (optional): https://caniuse.com/css-relative-colors
• Bugzilla URL (optional): https://bugzilla.mozilla.org/show_bug.cgi?id=1701488
• Mozillians who can provide input (optional): @emilio

Other information

• Shipped in Safari 16.4.
• Chrome bug: https://bugs.chromium.org/p/chromium/issues/detail?id=1447327&q=relative%20color&can=2
• WPT tests (2076 subtests) (most passed by Safari): https://wpt.fyi/results/css/css-color/parsing?label=master&label=experimental&aligned&q=relative
• CSSWG issue: Is relative color syntax ready to ship? [css-color] Is relative color syntax ready to ship? w3c/csswg-drafts#7978
• CSS WG issue: compat issues [css-color-4] [css-color-5] compat issues w3c/csswg-drafts#8943

[svgeesus]

Oh, relative color syntax is also supported in Lightning CSS, a server-side preprocessor written in Rust:

https://lightningcss.dev/transpilation.html#relative-colors

[svgeesus]

@emilio is there an official position?

[emilio]

I think we generally like the shape of RCS. cc @tlouw who has looked more into it.

I'd love more tests for the feature because just from a quick look a couple weeks ago Safari doesn't support currentColor in RCS, which seems like a big oversight and apparently nobody noticed?

I'd suggest a positive position, assuming the test-suite improves, and also assuming Tiaan agrees :)

[svgeesus]

Yeah looking at parsing/color-computed-relative-color.html there is no test for currentColor.

currentColor in RCS was discussed:

• [css-color-5] What should the behavior of the CSS Color 5 color functions be when passed currentcolor as <color>  w3c/csswg-drafts#6168
• [css-color-5] Does relative colors support current color? w3c/csswg-drafts#9238

[tiaanl]

The spec as it is now looks good. Only thing coming up for me during prototyping was the currectcolor issue, but that has been addressed now.

Implementation for this will start soonish.

[svgeesus]

@emilio could you review web-platform-tests/wpt#41933 which adds some RCS currentColor tests?

[tiaanl]

Having currentcolor in color-valid-relative-color.html would also help to make it clear that parsing and serializing it as specified value is valid.

[svgeesus]

Good point, I will add it.

[svgeesus]

Having currentcolor in color-valid-relative-color.html would also help to make it clear that parsing and serializing it as specified value is valid.

Added in web-platform-tests/wpt@9a43e48

[svgeesus]

@tiaanl could you review web-platform-tests/wpt#41933

[tiaanl]

@svgeesus Yup looks great. Thanks!

[svgeesus]

Okay well please say so as an approving review comment :)

[svgeesus]

@tiaanl thanks! @emilio when you get a chance?

[svgeesus]

Tests are all merged now, thanks to the reviewers.

Will this positive position be reflected on https://mozilla.github.io/standards-positions/ at some point?

[martinthomson]

Lots of noise here, I missed the two positive statements up-thread (and I suspect others did too). I'll defer to them as to whether this appears on a dashboard.

@emilio, @tiaanl, if you think that this needs a dashboard entry, can I request that you create a pull request?