LibWeb: Fix CSS tag seletor case sensitivity for SVG elements

[Psychpsyo]

For things like tag names, the CSS spec defers whether or not they are case-sensitive to the specifications these names come from.
In HTML, tag names are case-insensitive, but in SVG they are case-sensitive.
I'm not sure which of the two MathML falls under, but I also don't think we currently support that.

The test uses foreignObject, cause selecting those by tag name was impossible before this fix.

[AtkinsSJ]

From what I can find, anything based on XML is case sensitive. So SVG, MathML and XHTML all are. I don't know if we currently have a way to distinguish XHTML.

[Psychpsyo]

From what I can find, anything based on XML is case sensitive. So SVG, MathML and XHTML all are. I don't know if we currently have a way to distinguish XHTML.

We have, it is element.document().document_type() which returns either DOM::Document::Type::HTML or DOM::Document::Type::XML.
Of course, this only works in the whole-document case, but I don't think it is possible to nest XHTML documents in HTML ones, like with SVG, so that should be fine.

[Psychpsyo]

By which I mean "Yes, good point. Fixed the PR to take that into account."

EDIT: actually do not merge this yet, I found the relevant bit of HTML spec that clarifies this.

Ok, this should now be ready to merge, and with more tests.

[AtkinsSJ]

EDIT: actually do not merge this yet, I found the relevant bit of HTML spec that clarifies this.

For reference, you can turn a PR into a draft, or back again, which makes it really clear that it shouldn't be merged. (Because it disables the merge button!)