[css-scoping] [selectors] :scope and DocumentFragment / ShadowRoot #7261
Description
Relevant spec sections are:
- https://drafts.csswg.org/selectors-4/#the-scope-pseudo
- https://drafts.csswg.org/selectors-4/#scoping-root
- https://dom.spec.whatwg.org/#dom-parentnode-queryselector
- https://drafts.csswg.org/css-scoping/#host-element-in-tree
If I read the specs correctly:
Element.querySelectorAll(":scope > *")returns all the direct descendants of the given element. This is interoperable behavior and not ambiguous.Document.querySelectorAll(":scope > *")ought to work and return only the root element (the scoping root would be theDocumentnode, and it has only one child, which is the document element). Browsers are interoperable here but not following this reasoning. Instead,:scopeis the root element (so such a query returns the<head>and<body>elements, usually).DocumentFragment.querySelectorAll(":scope > *")ought to work and return the children of the fragment, because the scoping root is the fragment, and is a virtual scoping root. This works in Blink, but not in Gecko and WebKit (:scopeis undefined, so it falls back to the root element of the document, so it never matches anything).- In style sheets,
:scopeis always the root element of the document, because there's no defined scoping node, and that's the fallback. That's Gecko and WebKit's behavior. Blink in shadow trees considers the shadow root a:scope(https://crbug.com/1272434), but:scopedoesn't match the shadow host directly like:hostdoes.
This is all very unfortunate and inconsistent. It's clear that the spec is wrong for the Document.querySelector case, IMO (though the spec doesn't define Document as a potential "virtual" scoping root, it follows that if such should work for DocumentFragment it ought to work for Document as well?).
We should probably agree on the non-interoperable cases. If we wanted to make :scope behave consistently for Document and DocumentFragment, we ought to probably either:
- Make all direct children of the fragment match
:scope(rather than what the spec currently says / what Blink does), or. - Leave the scope node undefined (which is what WebKit / Gecko does), so it always matches the root element as a fallback.
For the stylesheets case, I think WebKit and Gecko follow the spec and the spec is somewhat reasonable. We have :host for shadow trees, and even tho defining :scope is possible, it might be a bit tricky, depending on how we define the above.
Thoughts?