[css-view-transitions-2] :active-view-transition-type() and reserved keywords interaction is not defined

[emilio]

I was implementing view-transition-types in gecko, and there are two tests that I had to reverse-engineer Chrome to pass:

• https://wpt.fyi/results/css/css-view-transitions?label=master&label=experimental&aligned&q=view-transition-types-reserved

AFAICT there's nothing in https://drafts.csswg.org/css-view-transitions-2/#the-active-view-transition-type-pseudo that would otherwise prevent those from matching:

• The ViewTransitionTypeSet can contain -ua- prefixed stuff and none just fine, it's just a set of case-sensitive strings.
• The :active-view-transition-type() selector doesn't restrict parsing of <custom-ident> to avoid including -ua- and none.

The way this behavior is implemented in Chrome and WebKit is at match time. You never consider a view transition type that starts with -ua- or is an ascii-case-insensitive match for none active. But that's not in the spec.

We have a resolution in #9534 (comment) which seems to indicate that :active-view-transition-type(none) should be invalid, but that's also not what's implemented, what's the intended behavior here?

cc @nt1m @noamr @vmpstr

[emilio]

Ah, #10706 has extra discussion that mentions not validating in JS, but it seems like the idea is that CSS.supports("selector(:active-view-transition-type(-ua-foo))") should return false?

[emilio]

It's also very weird that the check for -ua- prefix is ascii-case-insensitive since the type set is case-sensitive?

[noamr]

We have a resolution in #9534 (comment) which seems to indicate that :active-view-transition-type(none) should be invalid, but that's also not what's implemented, what's the intended behavior here?

This resolution is about the cross-document @view-transition rule, and that is spec'ed in https://drafts.csswg.org/css-view-transitions-2/#resolve-view-transition-rule (step 7).

I see in hindsight how the resolution wording can be ambiguous and led to this divergence between spec and implementation.
Changing the spec and WPTs to not accept -ua- and none in :active-view-transition-type() SGTM.