[css-pseudo] should all highlights have paired defaults?

[delan]

#6386 asked a bunch of questions about paired defaults, which is the compat behaviour where the UA default ::selection ‘color’ and ‘background-color’ (‘HighlightText’ and ‘Highlight’) are only used when neither were set by the author.

In particular, question 3 asks whether this should apply to the other highlights too, and this issue is meant to resolve that question. Currently ::target-text is the only pseudo with defaults for those properties (‘MarkText’ and ‘Mark’), so it’s the only pseudo that’s affected by this issue.

One reason to say no is that ::target-text doesn’t have the legacy of ::selection that made this rule necessary. One reason to say yes is consistency, since the highlights are already a complicated enough pile of exceptions without adding an exception for just one of them. Another is that the behaviour can be useful for the same reasons it is for ::selection, that is, avoiding situations where the author setting only one of the properties yields an illegible result in another browser with different defaults.

(see also: fantasai’s comment, my comment, emilio’s comment)

[astearns]

The bot missed adding the minutes. Here’s a hand-edited version:

RESOLVED: paired defaults apply to all highlights

The full IRC log of that discussion

Topic: Paired defaults
delan: This issue won't affect implementation complexity much, about what makes sense to authors
delan: some highlight pseudos have default background color and default color
delan: e.g. ::selection has default colors
delan: and ::target-text
delan: but it's not true for e.g. spelling/grammar error, where default styles add squiggly lines but don't change colors
delan: ::selection has special behavior called "paried defaults"
delan: rule is there for compat reasons
delan: means that those default colors only get used
delan: if neither color nor background was set by the author
delan: beside compat, has some other benefits
delan: prevents illegible combination of colors
delan: e.g. some browsers have light text for selections, and some have dark text
delan: so question is, should these apply to other highlight types?
delan: or only to ::selection?
florian: Point about default colors being relied on makes sense, so I would say yes it should apply to all
emilio: Gecko doesn't have this magic if the author specifies selection styles
1emilio: do you know the state of interop?
1delan: it's also in Gecko
1emilio: if you specify ::selection { background: green ; color: green; } we wouldn't magically infer the color
1delan: that's separate
delan: this is if the author only sets one of these properties, does it get the UA color or does it get transparent/currentcolor
delan: inverting is a separate issue
florian: If we don't do it we have a risk of browser-dependent illegible things, and that's bad
fantasai: I think propsoed resolution is, do we apply paired defaults to all highlights
fantasai: so do we have any objections? if not we can resolve
delan: so resolution is that they would apply to all fo the highights?
florian: we have delan, fantasai, and myself agreeing and nobody is pushing back
Rossen_: I'm not hearing any objections, so let's resolve
Rossen_: if anyone thinks about this we can come back and change if necessary
RESOLVED: paired defaults apply to all highlights

[delan]

#6665 assumed we would resolve in favour of ::target-text using paired defaults (see this thread), so no edits needed :)

Some highlight pseudo-elements should have paired default highlight colors—a default color and background-color provided by the UA that are either used or overridden together, see § 3.3.1 Paired Defaults. For ::selection they should correspond to HighlightText and Highlight, while for ::target-text they should correspond to MarkText and Mark.