Re: [webcomponents] Support Custom Pseudo-elements (#300)

I think custom pseudos are a good direction. Comments on some specific issues:

(1) It's critical for component authors to be able to whitelist the set of style properties that can be applied to an exposed named part.
(2) I'm somewhat partial to :part syntax because then we never have to worry about namespace collisions with future CSS built-in pseudos.
(3) I don't think prefixing is necessary. If pseudo names are reused between different custom elements, that's not a problem, because you can specify the element and part.
(4) I don't think @apply plus the increasing series of other features that need to come along with it is a replacement. The syntax is pretty inscrutable. I immediately understand what Philip's named part examples are doing, but I don't understand what his @apply + nesting proposal does.
(5) Having a nice and easy to understand syntax for consumers of a component is important. Unlike many other aspects of web components, this syntax will be used by many garden-variety web developers, not just authors of JavaScript frameworks.

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