Change local-name computation in DOM::validate_and_extract() to preserve
everything after the first colon in a qualified name — matching a recent
spec change made in https://github.com/whatwg/dom/pull/1455 (and
replacing a previous spec requirement to use the “strictly split”
algorithm, which resulted in throwing away any other part of the name
after any second colon the name might have).
For the qualified name “f:o:o”, this change now gives localName=“o:o”.
Otherwise, without this change, it’d instead give localName=“o”.
