Re: [w3c/webcomponents] Support Custom Pseudo-elements (#300)

> I anticipated this reaction, and I probably should have been more explicit about what it's doing: ::theme() is exactly the same, mechanically, as @apply. Literally, modulo some unimportant corners of the functionality, the two are doing precisely the same thing - letting you target a component arbitrarily far down in the shadow hierarchy.

I may not understand @apply very well. But it seems like @apply puts control in the hands of component authors for where stuff gets applied, and the expected API contract is that the client of the component provides some custom properties with the intended styles. But @theme applies style to all parts with a given name everywhere. And the component client has to decide whether to use @part or @theme. So the component's API contract is not just a specific name of the styling hook, but also the requirement to know whether they should use ::part or ::theme with it. 

Meanwhile, forwarding provides a completely component-controlled way of handling it where the client should always just say ::part. 

Components authored with a closed shadow DOM will have to be done with forwarding, and in that case ::theme is useless. So different kinds of components will have different API contracts for how to style them that are dependent on an authoring choice that shouldn't be relevant to this.

In brief, it seems like telling your users to use ::theme (or leaving the choice to them) is poor authoring practice, and explicitly forwarding part names is good practice. The fact that ::theme theoretically is similar in power in some sense to @apply is not a good reason to have it. Depth of styling should be controlled by the component, not the client of the component, and leaving it to client code adds a needless confusing decision.

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