Re: [webcomponents] [Shadow]: Style element should specify when to create and add style sheet in shadow DOM (bugzilla: 26850) (#56)

In
https://github.com/w3c/web-platform-tests/blob/master/shadow-dom/ShadowRoot-interface.html
`testStyleSheets()` hits this issue.

The test creates a `<div>` and attaches a shadow on it, then appends some `<style>`s
in the shadow, and checks `styleSheets.length`.
In the test, the shadow host `<div>` is never attached to anything, so these `<style>`s are not
connected to the document, thus the `styleSheets.length` could be 0, but the test expects 2.

(For here, let's assume "in a Document" in the discussion above means the shadow host is
appended to somewhere traversible from the document root.)

Clarification needed when style sheets are added to ShadowRoot,
1) at `<style>` is attached in a 'ShadowRoot''s tree (the wpt test assumes this)
2) at its attached `ShadowRoot` is attached in the document tree

2 makes sense in that the timing is `<style>` is effectively applied (i.e. affects actual rendering).

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https://github.com/w3c/webcomponents/issues/56#issuecomment-173818950