[w3c/webcomponents] Should <link> work inside Shadow DOM? (#628)

After https://github.com/w3c/webcomponents/issues/530, to keep `<style>` & `<link rel="stylesheet>` consistent, both works inside Shadow DOM. My question is, how about consistency with other link types (`rel`s)?
What about `author` or ekhm.. `import`?

I would like to indicate an author of the document fragment I attached to shadow root, as well as custom element definitions bulked with CSS in imports. I know imports are on hold, but is there a reason to exclude all links except `stylesheet`? Or to block loading external document fragments into shadow root?

They are excluded explicitly at http://w3c.github.io/webcomponents/spec/shadow/#dfn-inert-in-a-shadow-tree but I cannot find anything similar at https://html.spec.whatwg.org/. I'm not sure whether this is still to be upstreamed (https://github.com/w3c/webcomponents/issues/377#issuecomment-250693594) or this constraint was dropped.

[Current Blink implementation](https://github.com/dstockwell/chromium/commit/f0cecf3ac0c7adf7a7c78eeb02cc75d380a9ce4d) blocks all `<link>` elements except `stylesheet`  what [results in](http://jsbin.com/gigupif/edit?html,console,output) "HTML element `<link>` is ignored in shadow tree." warning, which is not so precise, as in fact not all `<link>` elements are ignored.

To draw my use-case: I use Shadow DOM for styling 3rd party content. So, not to break functionality, I attach HTML composition to Shadow Root.
https://starcounter.io/unobtrusive-styling-composing-3rd-party-html-content/
The problem is that, I would like to use custom elements (both vanilla and Polymers) which are delivered with CSS rules.

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