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

it's not just display though, from pre to post upgrade all kinds of things will change - shadow root contents, distribution, and all other style properties. Controlling FOUC will require care in any case, and for styles will involve applying styles to unresovled elements. `display: block` should reduce, though not eliminate, one of the more potentially jarring sources of FOUC, given that most elements want block.

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