Auto merge of #14047 - mskrzypkows:getElementsByTagName, r=nox

fix getElementsByTagName()

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Improved implementation of getElementsByTagName() in Document, properly pass 3 cases of https://dom.spec.whatwg.org/#concept-getelementsbytagname

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix  #11596 (github issue number if applicable).

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/script/dom/webidls/Document.webidl

@@ -28,8 +28,8 @@ interface Document : Node {
   readonly attribute DocumentType? doctype;
   [Pure]
   readonly attribute Element? documentElement;
-  HTMLCollection getElementsByTagName(DOMString localName);
-  HTMLCollection getElementsByTagNameNS(DOMString? namespace, DOMString localName);
+  HTMLCollection getElementsByTagName(DOMString qualifiedName);
+  HTMLCollection getElementsByTagNameNS(DOMString? namespace, DOMString qualifiedName);
   HTMLCollection getElementsByClassName(DOMString classNames);
 
   [NewObject, Throws]