In this Tag Set, the attribute-name-like construction “xmlns:%MATHML.prefix;” resolves to xmlns:mml, which is not an attribute, but a namespace pseudo-attribute. The value provides a prefix to use for the MathML elements. All namespace prefixes must be associated with a URL, and the prefix “mml” has been set to the URL for the World Wide Web Consortium (W3C) MathML 2.0 Tag Set or MathML 3.0 Tag Set.