What's the point of doing this change? I'd rather not diverge the behavior of various elements inside shadow DOM as much as possible.

If you have:

<link rel="alternate stylesheet" title="mystyle" href="...">
<custom-component></custom-component>

and <custom-component> has a <style title="mystyle"> inside its shadow tree, it will set mystyle as the preferred set and enable the alternate stylesheet in the document tree.

From 7.1:

"At the same time, it is an encapsulation abstraction, so it has to avoid affecting the document tree."

Is there something in the spec already that would cover this case?

Setting the preferred stylesheet set is defined in https://drafts.csswg.org/cssom/#add-a-css-style-sheet which talks about "document CSS style sheets" which are the ones reflected in document.styleSheets, which shadow dom style elements are not part of. Perhaps that's enough evidence that shadow dom style title should not set the preferred set, at least?

Yeah, although the spec should not mention it clearly, we should honor an encapsulation here.

This should be one of the instances which we should use the label html-dom so that we do not forget this issue in upstreaming Shadow DOM into HTML/DOM Standard.

FWIW, the Blink issue is https://crbug.com/588718 which is now fixed.

This should really be fixed in the CSSOM specification I think once we have the terminology in place. Paging @zcorpan.

Or maybe this is still defined in HTML? Hmm.

It's specified in the CSSOM spec, and as I wrote earlier, you can sort of deduce that this shouldn't apply to shadow dom stylesheets, but I don't think it was written with shadow dom in mind.

We should still update it to account for stylesheets within the Shadow DOM, no?

I think it would be still useful to clarify it in the shadow DOM spec.

Yeah, let me have another section for clarification, mentioning misc. such as script, iframe, title attributes of style and so on, as a tentative clarification before upstreaming.

CSSOM bug is https://www.w3.org/Bugs/Public/show_bug.cgi?id=29349