[css-scoping] context for evaluating selectors

The definitions of the :host, :host(), and :host-context() pseudo
classes in http://drafts.csswg.org/css-scoping/#host-selector all
refer to the selectors being "evaluated in the context of a shadow
tree".

However, I don't see a definition of what this means.

Tab and I discussed this briefly on IRC last week:
http://logs.csswg.org/irc.w3.org/css/2015-03-16/#e533427
where Tab gave the following rough definition:
  <TabAtkins> Not well! I wasn't sure what the right hooks were. But
  it should be "from a <style> or <link> inside a shadow tree, or in
  a DOM api (somehow) rooted in a shadow tree".
  ...
  <TabAtkins> I think that if you can start the selector in a shadow
  tree or on a shadow root, you're "in the context"

But this makes me wonder how one of the examples in the spec works.
http://drafts.csswg.org/css-scoping/#host-element-in-tree explicitly
says that one of the goals of these selectors is styling the shadow
host (rather than just selecting its descendants based on something
about it).

Given that, it's not clear to me how this intends the matching and
application of style rules to work.  What makes the selectors that
come from a style or link element inside the shadow tree be applied
when deciding what styles apply to the host element, when everything
seems to say that they apply only inside the shadow tree?

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
             Before I built a wall I'd ask to know
             What I was walling in or walling out,
             And to whom I was like to give offense.
               - Robert Frost, Mending Wall (1914)