[w3c/webcomponents] Respec errorrs (#874)

```
Duplicate definition(s) of 'import' at: 1.
Couldn't match "HTMLLinkElement" to anything in the document or in any other document cited in this specification: webidl. See how to cite to resolve the error at: 1.
Couldn't match "Document" to anything in the document or in any other document cited in this specification: webidl. See how to cite to resolve the error at: 1.
Couldn't match "custom element" to anything in the document or in any other document cited in this specification: custom-elements, webidl. See how to cite to resolve the error at: 1.
```

@marcoscaceres - can you take a look?

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