[css-fonts] Computed value of font stretch

[emilio]

https://drafts.csswg.org/css-fonts/#font-stretch-prop has:

For compatibility with [CSS-FONTS-3], getComputedStyle() serializes values that correspond to one of the font-stretch keywords as that keyword (instead of as a ).

However Chrome and Firefox don't do this, so it's clearly not needed for compat. Should we remove that clause from the spec?

cc @svgeesus @litherum

[emilio]

cc @graouts (we noticed this because of web-platform-tests/wpt#28315 caused some tests to fail in Firefox).

Probably while this is sorted out we could update the test to accept both serializations.

[jfkthame]

Note that there are a bunch of other WPT tests where this is also relevant, e.g.

https://wpt.fyi/results/css/css-fonts/inheritance.html
https://wpt.fyi/results/css/css-fonts/parsing/font-stretch-computed.html
https://wpt.fyi/results/css/css-fonts/variations/font-stretch.html

Depending what we resolve (always serialize as percentage? allow either percentage or keyword?) these may need updating; currently wpt.fyi shows a bunch of them failing in Safari because it uses the keyword serialization.

[css-meeting-bot]

The CSS Working Group just discussed [css-fonts] Computed value of font stretch, and agreed to the following:

• RESOLVED: font-stretch should serialize to a number/percentage, not a keyword

The full IRC log of that discussion

<myles> Topic: [css-fonts] Computed value of font stretch
<astearns> github: https://github.com//issues/6171
<fremy> emilio: the spec has a note that says that for compat reasons, computation of font-stretch should stay a keyword if it was one, instead of the percentage
<fremy> emilio: webkit doesn't do that
<fremy> emilio: tests either require one or the other, there is no concensus
<fremy> emilio: I would like to remove this note
<fremy> myles: there is no known issue I was working around
<fremy> dbaron: usually, we try to stick to the most compatible representation for compat
<fremy> emilio: returning the keyword makes it difficult for authors because they need to reverse compute the keywords
<fremy> emilio: font-3 used to report the keyword all the times
<fremy> myles: at least in webkit, we implemented font-stretch at the same time as the numerical values
<fremy> myles: so the span of time where we didn't support numbers for it was short
<fremy> myles: possibly not even a release
<fremy> dbaron: given that, I think it's fine to break the general principle
<fremy> astearns: ok, so the proposed resolution is to not used the font-stretch keywords in serialization
<fremy> emilio: and return a percentage
<fremy> astearns: ok, any objection to this?
<fremy> RESOLVED: font-stretch should serialize to a number/percentage, not a keyword