[w3c/webcomponents] HTML module should require a new top level element (#752)

Example:
```html
<!DOCTYPE html>
<module xmlns="http://www.w3.org/1999/xhtml">
 <!-- HTML Module stuff goes here -->
</module>
```

## See also:
- #645
- #742

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Received on Saturday, 19 May 2018 12:53:28 UTC