Re: [w3c/webcomponents] HTML Modules (#645)

Summary from the Tokyo face-to-face:

There was interest in this proposal, but a number of questions and details that need to be answered. Among them are how to specify the parser behavior, what elements are allowed in HTML modules, the stages of processing needed to build the exports, whether a Document or DocumentFragment is created, what realm is used, and whether a base URL is needed.

We need to highlight all the decision points and the pros and cons of each possible answer. To test out various choices, we need a list of concrete use cases. @annevk would like to see a matrix of choices vs use cases to highlight how well variations of the proposal meet them.

These are not complete lists, but some of what we discussed at the meeting:

### Use cases
- Import a template into an element definition in a JS file.
- Single file components: templates, imports and script for a component are in a single .html file
- Declarative custom elements: They need to be declared in an .html file.
 - Polyfills for declarative custom elements, which likely will be custom elements themselves, need to run in .html files.

### List of questions and possible answers

- How does an HTML module build it's exports? It needs to parse HTML and inline scripts, load transitive resources, etc.
- Should HTML modules be restricted to only module scripts?
- How do we specify (I'm a little unclear on some of the specifics that follow) differences in parsing behavior from the main document, ie, that scripts don't block parsing, etc. Anne's idea was to use the XHR parser, which parses an inert document and then post-processes to kick off certain actions.
- Which elements are handled in post processing?
- For elements not handled, do we throw to preserve forward compatibility if they're handled in the future?
- What APIs are available to script? ie, `document.write` is blocked in modules, what else?

@TakayoshiKochi @domenic @annevk @rniwa does this sound right?

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