Re: [whatwg/dom] Declarative Shadow DOM (#831)

I'm still somewhat on the fence about whether or not we should be able to refer to a pre-existing template so we can avoid repetition.

One of the *base* use-cases - getting "scoped styles" for a section of your page using the shadow DOM composition boundary without needing JS - is satisfied without that. If you're hand-authoring a no-JS page, you have to repeat all your structure for each element; this doesn't change anything about that, it just adds a little bit more text to each repetition to establish the boundary.

Another satisfied base use-case is shipping server-rendered HTML using shadows that'll be hydrated into full JS-driven custom elements later.  You can write custom elements (without having to repeat the contents each time) on your server, then serialize them out into this form; compression should take care of most of the cost of repetition, and post-parsing DOM sizes are comparable.

The use-case not satisfied is wanting to get the less-structural-repetition benefit of a custom element without requiring JS if all you're doing is filling in some DOM and nothing else. That's a reasonable case, I think! But also a less important case than the two I mentioned above. I think if we go without that for now, we're not blocking ourselves from having such a solution later, such as having a `<template use=#id></template>` that lets it refer to templates already in the DOM? And avoiding that for now lets us skip some more complex scenarios, making the MVP here easier to define.

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