[selectors-4] misleading/incomplete Note: clarify that <forgiving-selector-list> does not work as expected with pseudo-elements

[claudepache]

The Note in Section 17.1 <forgiving-selector-list> and <forgiving-relative-selector-list> says:

Note: Style rules still use the normal, unforgiving selector list behavior. <forgiving-selector-list> is used in some functions, like :is(), which are similarly generic. Although it does have some minor implications on specificity, simply wrapping a style rule’s selector in :is() effectively "upgrades" it to become forgiving.

There is however a much more serious implication than specificity when the selector list targets pseudo-elements. The following won’t work as naïvely expected (js-bin live example):

:is(button, input[type=button], input[type=submit], ::file-selector-button) {
    background: fuchsia;
}

[tabatkins]

The "implication" is just that pseudo-elements aren't valid in :is(), right?

[Loirooriol]

That's already implied in the spec:

Pseudo-elements cannot be represented by the matches-any pseudo-class; they are not valid within :is().

#2284 is about making them valid (deferred to L5)

Forbidding pseudo-elements in forgiving selector lists will also cover :has(), which seems good.

[claudepache]

Just for clarity. Both specificity and pseudo-element handling of :is() are correctly mentioned in the spec. My issue is that (unless #2284 is fixed) the specific Note in section 17.1 is currently misleading, because developers will try to wrap some selector list containing some partially supported pseudo-element with :is().

[tabatkins]

All right, text amended to clarify that you can't "upgrade" a selector list that contains pseudo-elements.