Re: [csswg-drafts] [css-backgrounds][css-animations] Should background-* and animation-* longhands computed values serialize using the same truncation rules? (#12791)

The CSS Working Group just discussed `[css-backgrounds][css-animations] Should background-* and animation-* longhands computed values serialize using the same truncation rules?`, and agreed to the following:

* `RESOLVED: no change, update WPT to match the spec's expectations and see if there's web compat issues`

<details><summary>The full IRC log of that discussion</summary>
&lt;ydaniv> weinig: this is about whether compat will hold up here, animations/background longhands are coordinated lists<br>
&lt;ydaniv> ... wanted to know if we want to resolve on compat serializations<br>
&lt;ydaniv> TabAtkins: in one of the properties having one tramps the others, in the other it's not<br>
&lt;ydaniv> ... I just care about having a consistent behavior<br>
&lt;ydaniv> weinig: not sure even all browsers do that, it's just what WPT test<br>
&lt;ydaniv> ... should be as round-trippable as possible<br>
&lt;ydaniv> ... I would be sticking with what the spec says should be correct<br>
&lt;ydaniv> emilio: seems FF is already doing that<br>
&lt;ydaniv> ... so probably compatible<br>
&lt;emilio> https://wpt.fyi/results/css/css-backgrounds/parsing/background-repeat-computed.html<br>
&lt;ydaniv> weinig: ok, thanks<br>
&lt;astearns> ack fantasai<br>
&lt;ydaniv> fantasai: I support what TabAtkins said, don't have opinion on which way<br>
&lt;ydaniv> ... backgrounds is a canonical example for this<br>
&lt;ydaniv> weinig: I think background is the one where it does not match the spec's behavior<br>
&lt;ydaniv> ... but in FF seems ok<br>
&lt;astearns> ack dbaron<br>
&lt;fantasai> s/this/coordinating list property group, spec doesn't use the term because it predates the term/<br>
&lt;ydaniv> dbaron: one thing that can happen, gCS and computed value are different things<br>
&lt;emilio> q+<br>
&lt;fantasai> s/on which way/on which way as long as Web-compatible/<br>
&lt;ydaniv> ... I would not like the truncation to be different, but it is today in some implementations so should be careful<br>
&lt;ydaniv> ... I think my prefs aggree with weinig, so that both computed value and gCS return the same value<br>
&lt;emilio> q-<br>
&lt;ydaniv> weinig: they should likely match, and I'll ensure on WPT we have something we're explicitly testing<br>
&lt;ydaniv> dbaron: there were some implementations that did weird thing to the computed value, don't know if these are still around today<br>
&lt;ydaniv> astearns: should we resovle on what we wish to see? and look for compat concerns? Or make smaller viable changes?<br>
&lt;ydaniv> weinig: this mostly comes down to changing background and mask-image in WK and CH, and update tests<br>
&lt;ydaniv> ... probably tests first<br>
&lt;ydaniv> astearns: updating the tests makes sense if we updated spec<br>
&lt;ydaniv> weinig: the spec is correct<br>
&lt;astearns> ack dbaron<br>
&lt;ydaniv> dbaron: side comment, if we do find a compat problem here, I'd like to keep the background and mask matching, then to try and make them match others<br>
&lt;fantasai> +1 dbaron<br>
&lt;ydaniv> +1 dbaron<br>
&lt;ydaniv> PROPOSED RESOLUTION: no change, update WPT to match the spec's expectations and see if there's web compat issues<br>
&lt;ydaniv> astearns: obejctions?<br>
&lt;ydaniv> RESOLVED: no change, update WPT to match the spec's expectations and see if there's web compat issues<br>
</details>


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Received on Wednesday, 11 March 2026 16:00:39 UTC