[css-text-decor-4] Allow to interpolate between `auto` and length values in `text-decoration-trim`

[SebastianZ]

In https://bugzilla.mozilla.org/show_bug.cgi?id=1997043, @yisibl noted that authors might want to animate between auto and <length> values.

Therefore, we should consider defining how they can be interpolated.

Sebastian

[yisibl]

text-decoration-thickness: auto/from-font and text-underline-offset: auto also has a similar issue.

[Loirooriol]

Maybe calc-size()? Or can auto be resolved into a length at computed-value time?

[jfkthame]

The spec for auto says

The UA chooses a trim amount that ensures that if two identical underlined elements appear side-by-side they do not appear to have a single underline

In the current Gecko implementation, we simply resolve auto to a small (font-size-relative) length, which can easily happen at computed-value time (and enabling animation between this and <length> values is trivial).

I can imagine a more sophisticated implementation of auto that would apply a non-zero amount of inset only when an adjacent element has decoration line that would abut the current one, which might make it trickier, but the spec doesn't currently call for this.

(If the description of auto said something more like "if two identical underlined elements appear side-by-side, the UA applies a trim amount...", there would be a clear argument that it should be context-dependent.)

[SebastianZ]

@jfkthame I saw that you already have a patch to allow animating between auto and <length> values. So I add this to the agenda that we reflect this in the spec.

Sebastian

[jfkthame]

Note that the gecko patch isn't really ready at this point.... in particular, Emilio pointed out that animating from auto to auto would result in the value being resolved to a pixel length, which may not be the expected result(?) So yes, I experimented a bit, but may not be ready to land this.

[Loirooriol]

I think the options are:

• auto computes to some length
• auto computes as-is, but animates as some length (like font-style: normal animating as oblique 0deg)
• auto animates as-is, but we can represent interpolations between auto and a length (like calc-size())
• auto can't be interpolated with lengths