Re: [whatwg/dom] Composed tree infrastructure (#725)

CSS's tree needs are:

1. "tree of trees" - a DOM tree, with shadow hosts included and the shadows accessible. This is used for Selectors, as they only apply to a single tree context at a time, but need to be aware of ::slot() pseudos on shadow hosts, and the qualities of the host element and its ancestors for :host() and :host-context(). 
 For CSS purposes, pseudo-elements are attached at this point.
2. "flat tree" - the above fully flattened, with shadows replacing light and slots filled in. This is also the "element tree" [mentioned in Display](https://drafts.csswg.org/css-display/#intro).  The conversion from DOM to flat tree [is defined in Scoping](https://drafts.csswg.org/css-scoping/#flattening).
3. box tree, and fragment tree, and some other trees that are all very CSS-specific

Where does this "composed tree" you mention fit in?

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