Re: [webcomponents] [Custom Elements]: Custom elements should be display: block by default (#224)

Hmm, that is a pretty good argument. Just to clarify for everyone:

1. means that `<x-foo>` goes from `display: inline` to `display:block` when you register `x-foo`.
2. means that `<x-foo>` is always `display: block`, even if you never register `x-foo`.

I agree that (2) is backward incompatible and (1) is pretty horrible. I can't see any way to make this work.

Maybe the best solution we can offer authors is some kind of selector like `:potentiallycustom` that matches any tag name with `-` in it, so they can add a one-line `:potentiallycustom { display: block; }` to their stylesheets. (This is different from `:unresolved` since it would still work after upgrading.)

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