[css-conditional-5] @supports elements queries

[NagayamaToshiaki]

There have been not much changes or addition of elements recently, but inability of querying the support of HTML element is not handy.
so I would like to issue the new specs that would be written like this:

@supports element(foo) {
    foo {
        line-height: 1.5;
    }
}

[cdoublev]

<foo style="color: red">Red</foo>

The above markup renders Red with the red color in most (all?) browsers, even when foo is not declared as an HTML custom element (web component). I think it would be hard to validate an element type in CSS, which can also apply in XML documents, for exemple.

Also, because element() allows an author to use an element in the document as an image, it would be confusing to get different behaviors (feature query and image source) in @supports and eg. background-image.

[svgeesus]

I agree that the proposed syntax has an unfortunate clash with the element() notation.

More concerning though is that we would need a crisp definition of what it means to support an element. Simply parsing it and not throwing it away? Something more, surely, but what exactly?