HTML
HTML: "http://www.w3.org/1999/xhtml"
MATHML
MATHML: "http://www.w3.org/1998/Math/MathML"
SVG
SVG: "http://www.w3.org/2000/svg"
XLINK
XLINK: "http://www.w3.org/1999/xlink"
XML
XML: "http://www.w3.org/XML/1998/namespace"
XMLNS
XMLNS: "http://www.w3.org/2000/xmlns/"
All valid namespaces in HTML.