Re: [webcomponents] Add "why upgrades" document to proposals section (#112)

@annevk I think parser special-cased elements are additive to upgradeable elements---you can have both. Not in the same element, sure. But I think authors are much more interested in progressively enhanced non-specially-parsed elements than they are in only-usable-after-loading-definitions specially-parsed elements. For the minority who do want the latter, we can definitely add that functionality, if they are willing to give up progressive enhancement.

This does match how user agents work at the moment, if you conceptualize the built-in element definitions being already loaded before parsing happens. I'm not sure what you mean by how it could constrain new features, though.

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