[whatwg/dom] Match querySelector for pseudo elements (::slotted()) (#463)

(See [crbug.com/710797](https://crbug.com/710797) for the background)

The problem is, that `shadowRoot.querySelector('::slotted(div)')` (or `querySelectorAll`) will not return anything even when the context object (the `shadowRoot`) contains `<slot>` and a `<div>` is slotted to it.

Demo:
http://jsbin.com/jafetekipi/edit?html,js,output
(currently both Blink and WebKit fail)

MDN document of [Document.querySelector()](https://developer.mozilla.org/en-US/docs/Web/API/Document/querySelector) says:
> CSS Pseudo-elements will never return any elements, as specified in the [Selectors API](https://www.w3.org/TR/selectors-api/#grammar)

Note that this "Selectors API" links to Selectors API level 1 (an old spec).
The latest draft as of today ([Selectors Level4](https://drafts.csswg.org/selectors-4/)) doesn't say about how `querySelector()` should work for pseudo-elements, and neither does DOM spec.

In the past, `::before` or `::first-line` etc. never matched any real element in DOM tree, but `::slotted(div)` can match a real element in DOM tree, even though it is in a different tree.

Here I see the following problems in the current specs:

1. DOM spec isn't clear about whether `querySelector` can match some pseudo elements (`::slotted()`).
2. DOM spec isn't clear about whether `querySelector` can return an element outside the node tree of the context object.
3. `::slotted()` is a pseudo element, but can point to a real element.

For 3, this is not a matter of DOM spec. The rationale for `::slotted()` being a pseudo element is that it points to an element in another tree, and we wanted to restrict the selector in parentheses to a simple/compound selector (not allowing complex selector). If it were a combinator, any selector would be allowed to the right side.

Note that issue 2 is also relevant to
https://github.com/w3c/webcomponents/issues/78 (Support `>>>` combinator in static profile).

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/issues/463