[selectors-4] Add :defined pseudo-class

[spectranaut]

[selectors-4] Add the :defined pseudo-class as discussed in #2258.

The original definition has been moved verbatim from the html spec and a note and examples added. PR to remove the definition from the html spec from WHATWG.

A test for this definition has previously been submitted: ewilligers/web-platform-tests@92a561a

Worked on with @zcorpan and @isaacdurazo.

[annevk]

See my comments at whatwg/html#4433 (comment). I don't think a change to how we do layering between CSS and HTML has been discussed so this specification should probably define this header more abstractly while leaving the actual matching requirements for DOM/HTML to HTML.

[zcorpan]

@annevk ok. Per the meeting discussion in #2258 (comment)

<dael> Rossen: If this has impl in various UA and since it's a selector makes sense as part of selectors unless we're against it in the way it's defined. We should accept it and refine
<dael> florian: Sounds good
<dael> Rossen: Other comments or Objections to adding :defined to selectors L4?
<dael> RESOLVED: Add :defined to selectors L4

As I understand this, the WG resolved to take the spec as-is (for now). cc @atanassov

Do we want this to follow the model of e.g. :link instead?

[annevk]

I think either Selectors embraces DOM/HTML and is defined on top of that data model or it keeps things abstract (which is shaky btw, there's some open issues around that) as it tries to do today. Doing both is too confusing though.

[fantasai]

I agree with @annevk and I believe the resolution was to add it at the same level of definition as e.g. :link etc, as he expects.

[fantasai]

[Marking needs work per @annevk's comments.]

[zcorpan]

I've changed this so that it has a high-level definition in Selectors, and has the examples be specific for HTML.

[zcorpan]

There are some tests in wpt for :defined already:

$ git grep ":defined"
css/selectors/invalidation/defined.html:    <title>CSS Selectors Invalidation: :defined</title>
css/selectors/invalidation/defined.html:    <meta name="assert" content="This tests that the :defined selector is effective">
css/selectors/invalidation/defined.html:      #a1:defined {
css/selectors/invalidation/defined.html:      :defined + #b1 {
css/selectors/invalidation/defined.html:      :defined > #c1 {
css/selectors/invalidation/defined.html:      div + :defined + * #d1 {
css/selectors/invalidation/defined.html:      }, ":defined selector is effective");
custom-elements/Document-createElement.html:    assert_false(instance2.matches(':defined'));
custom-elements/pseudo-class-defined.html:  :defined { color:${defined}; }
custom-elements/pseudo-class-defined.html:  :not(:defined) { color:${not_defined}; }
custom-elements/pseudo-class-defined.html:  // Test ":defined" changes when the custom element was defined.
custom-elements/pseudo-class-defined.html:    assert_equals(element.matches(':defined'), expected, 'matches(":defined")');
custom-elements/pseudo-class-defined.html:    assert_equals(element.matches(':not(:defined)'), !expected, 'matches(":not(:defined")');
custom-elements/pseudo-class-defined.html:  }, `${description} should ${expected ? 'be' : 'not be'} :defined`);

[zcorpan]

Ping @fantasai @tabatkins :)

[tabatkins]

Ah sorry for the delay.

Hmm. Overall this is fine, but now it sounds really abstract, to the point that I don't think I could figure out what it meant if I didn't already know about custom elements and their definitions.

I'll do a little tweaking and then accept it.

[zcorpan]

Thanks @tabatkins!

[bkardell]

If we get to more than just a single global registry, does this still hold up?

[zcorpan]

@bkardell this says it's up to the host language to say what matches :defined. For HTML this is https://html.spec.whatwg.org/multipage/semantics-other.html#selector-defined - that could be changed in HTML, and SVG could say something else, without needing updates to the Selectors spec.