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This is one of the parts of the draft that we may not end up doing as-is, depending on what else happens in the shadow DOM styling story.
Link to specification for reference: http://dev.w3.org/csswg/css-scoping-1/#shadow-pseudoelement
related issue: Implement >>> Selecting Through Shadows Selector for access into shadow root - https://bugzilla.mozilla.org/show_bug.cgi?id=1117572
I think this might belong in DOM :: CSS Object Model - please correct if I am mistaken.
If it's for implementing the pseudo-element in selectors, CSS Parsing and Computation is better.
To clarify, "CSS Parsing and Computation" is about parsing CSS and figuring out the values of CSS properties. "DOM: CSS Object model" is about scriptable reflections of CSS objects (rules, declarations, and so forth). If no JS is involved, it's not a "DOM: CSS Object model" issue.
It is also should be implemented in JS (such 'querySelector' and perhaps 'closest').
Those automatically pick up changes to selector matching, generally. But for the pseudo-element case they may need changes, yes, since right now those never match pseudo-element stuff. That would probably be a separate bug, and putting that one in CSSOM may make sense.
Looks like the shadow-piercing descendant combinator >>> is the replacement for the ::shadow pseudo-element. Boris, should this bug be marked as WONTFIX in favor of bug 1117572 or is it still too early? Sebastian  https://drafts.csswg.org/css-scoping-1/#selectordef-shadow-piercing-descendant-combinator
As-is this is INVALID since the standard changed. It's also unclear whether we want to implement >>> at this time (I'll comment on the other bug).
Regarding comment 10 I remove the ni for Boris. Sebastian