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xmlns:mml MathML Namespace Declaration

This is not an attribute, but the 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 Tag Set. The same namespace is used for both MathML 2.0 and MathML 3.0.
FIXED (not settable) on elements: <article>, <mml:math>
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 @xmlns:mml has a fixed value which may not be changed in the document.
Tagged Sample

@xmlns:mml namespace at the article level 

<article
  xmlns:mml="http://www.w3.org/1998/Math/MathML"
  xmlns:xlink="http://www.w3.org/1999/xlink"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  article-type="research-article" dtd-version="1.3"
  specific-use="migrated"
  xml:lang="en">
 <front>...</front>
 <body>...</body>
 <back>...</back>
</article>
Related Resource
For a discussion on namespaces used in JATS, see Namespaces and MathML.