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

Wouldn't a global rule like `:potentiallycustom { display: block; }` mean that elements that *want* to be `display: inline-block` will still cause FOUC? Same thing for elements that want to be `flex`, etc. If you're using custom elements from mixture of sources, there's no way to anticipate the `display` setting desired by each element. We agree that `display: block` is very common, but it's not universal.

FWIW, in practice nearly every element we create includes a shadow tree, so to prevent FOUC we end up needing to hide unresolved elements anyway. So point #1 hasn't been an issue for us. By the time the element is visible, it's applied its desired `display` setting.

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