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The spec says: On getting, the attribute must return a StyleSheetList sequence containing the shadow root style sheets. The term "shadow root style sheets" is not defined anywhere. In fact, the string "sheet" does not appear anywhere else in this spec. This needs to actually be defined, with particular attention to older shadow roots (e.g. do things in older shadow trees even load/parse their stylesheets?).
And in fact, https://bugzilla.mozilla.org/show_bug.cgi?id=1098270#c18 has at least one other case that should be considered.
There is already a bug for this: https://www.w3.org/Bugs/Public/show_bug.cgi?id=26850
That's not the same bug. There are two separate issues: 1) When do stylesheets get created for style/link elements? That's bug 26850. 2) What appears in the shadow root's .styleSheets and in what order? That's this bug. Figuring out bug 26850 is probably a prerequisite for resolving this one. But once that's done, this bug will still need to define which elements' stylesheets appear in the list, and the order.
Moved to https://github.com/w3c/webcomponents/issues/70