[webcomponents] What DOM content category do custom elements belong to? (#454)

A user of one of our component library has filed an [issue](https://github.com/basic-web-components/basic-web-components/issues/20) indicating that a [popular HTML minifier](https://github.com/kangax/html-minifier) is not minifying HTML with a custom element as expected. The specific repro case concerns a `<button>` that contains a custom element. The report suggests that perhaps the minifier only supports a whitelist of element types inside a button.

I'm less concerned with this specific issue than the general problem of tools (preprocessors, validators, etc.) that currently make assumptions about where certain element types are permitted. In order to facilitate the filing of issues/PRs on those tools, it might help to have some some definitive statement relating custom elements to the [kinds of content](http://w3c.github.io/html/dom.html#kinds-of-content) defined in other HTML specs.

I don't know what the correct content categorization for custom elements would be. Perhaps [flow content](http://w3c.github.io/html/dom.html#kinds-of-content-flow-content)? That is, they can appear essentially anywhere in the body. Or perhaps they should be defined as a new content type ("custom content") that's a proper subset of flow content.

To be clear, I don't have any opinion here. I'm just looking for a statement on this point to help convince tool maintainers to accommodate custom elements.

Does the custom elements spec already contain such a statement? I looked but couldn't find one.

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