[webcomponents] Interaction between CSS and composed tree needs to be better defined (#305)

Section 2.4 defines composed tree but its purpose and use isn't clarified in the section.

Also the following text
> Unless an element is in a document composed tree, the element must not appear in the formatting structure [CSS21] nor create any CSS box.
refers to "formatting structure" but this is a concept defined in a non-normative section of [CSS2.1](http://www.w3.org/TR/CSS2).

I think, we want to define the composed tree as the [document tree](https://drafts.csswg.org/css2/conform.html#doctree) in CSS instead.  Otherwise definitions os sibling, ancestor, etc... in the CSS world wouldn't make sense.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/305