This is an archived snapshot of W3C's public bugzilla bug tracker, decommissioned in April 2019. Please see the home page for more details.
For the moment, I intend to punt this completely and use CSS Variables to provide specific styling hooks in the shadow DOM, but it's still worth looking again at the "pseudo".
http://dvcs.w3.org/hg/webcomponents/rev/310a40f0ade1
Also http://msdn.microsoft.com/en-us/library/windows/apps/hh453342.aspx
Developers requested that on many occasions. CSS Variables isn't enough.
http://dvcs.w3.org/hg/webcomponents/rev/b2e59575a46b
http://dvcs.w3.org/hg/webcomponents/rev/71893ed10402