[webcomponents] Unclear on how to define :defined in CSS + HTML (#439)

@TabAtkins, could use your help here.

Here is what I have currently: https://w3c.github.io/webcomponents/spec/custom/#the-defined-element-pseudo-class-defined

It's pretty simple, but that's fine. What I'm confused about is how pseudo-classes get split between Selectors and HTML. E.g. compare:

- https://drafts.csswg.org/selectors-4/#enableddisabled
- https://html.spec.whatwg.org/#selector-enabled

Is the idea that we should have Selectors (or some other CSS spec) say something like

> The :defined pseudo-class represents elements that are defined as having behavior by the host language. For example, in [HTML], ...

and then in HTML we should say

> The :defined pseudo-class must match any element that is [defined](https://w3c.github.io/webcomponents/spec/custom/#dfn-defined-element)

? Is Selectors the right place for this? What heading would it go under? Maybe "Tree-Structural pseudo-classes"?

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