Re: [webcomponents] Disallow attachShadow(...) for some elements. (#110)

@rniwa it seems there's a large number of HTML elements it would be totally fine to use shadow DOM on. `<h1>` and friends, `<section>` and friends, `<var>` and friends, `<blockquote>`, `<div>`, `<span>`... It makes sense to avoid controls for now and anything with somewhat weird semantics (e.g., `<template>`), but in general I think we should allow it.

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