[css-text-decor] Specify a limit for ink-skipping dilation

[drott]

https://drafts.csswg.org/css-text-decor-4/#text-decoration-skip-ink-property

"The UA must also skip a small distance to either side of the glyph outline."

auto
UAs may interrupt underlines and overlines where the line would cross glyph ink and to some distance to either side of the glyph outline. UAs should consider the script of the text (see note below) when determining whether to apply ink-skipping behavior to a given range of content.
all
UAs must interrupt underlines and overlines where the line would cross glyph ink and to some distance to either side of the glyph outline.

I suggest to have a clearer definition of "small distance" and "some distance" to either side and introduce a capping at a maximum in skipping dilation.

See web-platform-tests/wpt#30788 (comment) for an example. If the ink skipping dilation is strictly proportional to the thickness without a limit to the sides, adjacent glyph underlines may completely disappear.

[fantasai]

I think this is fundamentally a quality-of-implementation issue, and I'm not sure what kind of limit would be reasonable to spec given I don't think we have a definitive answer to how this should be done. If you have some suggested wording, happy to consider it, though. :)

[fantasai]

(That said, I see no reason why the cited test isn't valid per spec as-is.)