xmlns:mml MathML Namespace Declaration

This is not an attribute, but the namespace pseudo-attribute. The value provides a prefix and URI 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 Tag Set.
FIXED (not settable) on many elements; click for list and usage
Value Meaning
http://www.w3.org/1998/Math/MathML The prefix “mml” has been set to the URL for the W3C MathML Tag Set.
Restriction This attribute has a fixed value which may not be changed in the XML document.
Related Resource
For a discussion on namespaces used in STS, see Namespacing NISO STS Elements.