get namespaceURI() derived the namespace from the tag name and only ever
recognised the <svg> root, so every element parsed inside an <svg> subtree
reported the XHTML namespace. get href() gates the SVGAnimatedString
reflection on the SVG namespace, so for parsed content the gate never
opened: href returned a plain string and .animVal/.baseVal were undefined,
making new URL(el.href.animVal) throw. Only createElementNS-built elements
worked, because createElementNS records _ns explicitly.
The tree builder already applies the HTML5 foreign-content rules, so
QualName.ns is correct for parsed SVG; it just was not exposed to JS. Add a
namespace_uri DOM op and read it in get namespaceURI(), keeping _ns as the
override and the old tag-name heuristic as the fallback when no element name
is recorded. Namespaces are immutable, so the result is cached like _lname.
Fixes#484
ulises2k
3a4e3f7d7d