[w3c/webcomponents] Polyfilling custom elements with native Shadow DOM support (#534)

I'm looking for guidance on how I should expect to use polyfilled custom elements in browsers that ship native Shadow DOM but lack native Custom Element support (i.e. Safari 10).

I realise this borders on somebody-else's-problem - go ask WebKit, go ask WebComponentsJS, etc - but I want to know if this is something that's been considered at the spec level and if there's any guidance on the best way to get around this problem.

Since Custom Element polyfills use Mutation Observers to upgrade elements that are created through the HTML parser (innerHTML or already in the document when registered), how should we handle this? If no native custom element support is available, should we force a polyfill for Shadow DOM so they can work?

Is there practical value in shipping Shadow DOM support without Custom Element support when it breaks existing polyfills?

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