[css-pseudo] [css-highlight-api] defaults and inheritance for ::spelling-error + ::grammar-error + ::highlight

Problem 1: UA default highlight colors

(this problem turned out to effectively be the same as #6774)

If I understand conventional behaviour and #highlight-ua-styles correctly, the intended default behaviour of ::spelling-error and ::grammar-error is to decorate the highlighted text without changing the foreground color.

~~Similarly, if I understand #6375 correctly, the intended default behaviour of an unstyled ::highlight is to yield no visible change in rendering, or at least not change the foreground color.~~

~~But given the following statements…~~

Highlight pseudo computed styles inherit all properties from the same highlight’s styles in the parent, eventually defaulting to initial values (#highlight-cascade). Used values can differ due to paired cascade ([css-pseudo] clarify paired-cascading behavior #6386).
~~color:currentColor is the only way to grab the foreground color from lower layers, that is to say, “don’t change the color compared to if this text was unhighlighted” (#highlight-text).~~
~~The initial value of color is a UA-defined ordinary color (css-color-3) or CanvasText (css-color-4), not currentColor.~~
There are no recommended UA default highlight colors for ::spelling-error + ::grammar-error (#highlight-ua-styles), be they magic used colors via paired cascade or just declarations in the UA stylesheet, and UA default highlight colors are forbidden for ::highlight as of [css-highlight-api-1] Specify that custom highlights have no UA styles #6375 #6534 (#default-styles).

~~…then by default, these three highlight pseudos will effectively reset the foreground color of highlighted text to initial (usually black), which is probably not what we want.~~

Problem 2: custom highlight inheritance clause

As of #6534, #default-styles says:

A custom highlight pseudo-element inherits the styles of its originating element.

I don’t think this really makes sense, because taken at face value, it essentially clobbers highlight inheritance (#highlight-cascade) in favour of the legacy behaviour (like how ::selection currently works in most impls).

Possible solutions

(see #6774 for solution to problem 1)

I think #highlight-ua-styles and #default-styles should explicitly recommend (or require) UA defaults for these three highlight pseudos that behave like color:currentColor. I believe the defaults should come from magic paired cascade, not UA stylesheet rules, to keep paired cascade as consistent and predictable as possible.

~~No change is needed for the other highlight color (background-color), because its initial value is transparent.~~

I think the spec text in Problem 2 should be removed.

Thanks for pointing this out!

For custom highlights at least, the original intention of the custom highlight #default-styles was indeed for an unstyled ::highlight to yield no visible change in rendering. I had written the spec text with the legacy ::selection behavior in mind, so it doesn't fit with the new #highlight-cascade behavior.

Maybe we could change #default-styles to something like the following, borrowing some language from #highlight-text:

If the author does not specify a custom highlight pseudo-element's color, the text is painted using the color of the next highlight pseudo-element layer below, or if none exists, the colors that would otherwise have been used (those applied by the originating element or an intervening pseudo-element such as ::first-line or ::first-letter). If the author does not specify a custom highlight pseudo-element's background-color, the UA must paint the highlight pseudo-element's background using the value 'transparent'.

fix problems with ::{spelling,grammar}-error + ::highlight (w3c#6779)

I had written the spec text with the legacy ::selection behavior in mind, so it doesn't fit with the new #highlight-cascade behavior.

Fair enough, that makes sense. I think that proposed wording is a good start, but I think it fails to account for the special relationship between ‘color’ and ‘background-color’ (paired cascade), and suffers from the same problems as “specified by the author” does in #highlight-cascade (#6386).

e6ec959ad9fb3 attempts to word this in a way that builds on the clarifications I’ve proposed in #6665, what do you think?

Ah, I'd missed that #6665 was also addressing this for custom highlights. It looks great to me!

@frivoal

We probably don’t need agenda time for this if @frivoal and/or @fantasai could review the PR

Highlight API: Don't paint unstyled custom highlights

Bug 1754327 [wpt PR 32759] - Highlight API: Don't paint unstyled cust…

Highlight API: Don't paint unstyled custom highlights

Bug 1754327 [wpt PR 32759] - Highlight API: Don't paint unstyled cust…

[css-pseudo] clarify paired cascade (#6386 + #6779 + #6774 + #7837) (#…

#6665 makes the edits for these questions.

Bug 1754327 [wpt PR 32759] - Highlight API: Don't paint unstyled cust…

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[css-pseudo] [css-highlight-api] defaults and inheritance for ::spelling-error + ::grammar-error + ::highlight #6779

Problem 1: UA default highlight colors

Problem 2: custom highlight inheritance clause

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[css-pseudo] [css-highlight-api] defaults and inheritance for ::spelling-error + ::grammar-error + ::highlight #6779

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Problem 1: UA default highlight colors

Problem 2: custom highlight inheritance clause

Possible solutions

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dandclark commented on Nov 17, 2021

delan commented on Nov 18, 2021

dandclark commented on Nov 18, 2021

astearns commented on Dec 7, 2021

delan commented on Oct 6, 2022

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