If an ascent is in light DOM and descendant is in shadow, descendant selector doesn't work.
*** Bug 89169 has been marked as a duplicate of this bug. ***
I'm not sure this is a desired behavior. I believe this is sometimes useful though...
The spec is quite specific on this point: <http://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#styles>
"Even when the apply-author-styles is set, the selectors still must not cross the shadow boundary per scoping constraints. In other words, with apply-author-styles set, the document CSS rules only match wholly inside or outside of the shadow tree."
This is the wrong forum for a spec bug; it should be taken up in the W3C.