2.2.49 [W3C-HTML51] Section 9.3 Serializing XHTML fragments
C0083: When XHTML documents are serialized, prefixes and namespace declarations are adjusted as needed
The specification states:
9.3 Serializing XHTML fragments ... In both cases, the string returned must be XML namespace-well-formed and must be an isomorphic serialization of all of that node's relevant child nodes, in tree order. User agents may adjust prefixes and namespace declarations in the serialization (and indeed might be forced to do so in some cases to obtain namespace-well-formed XML). User agents may use a combination of regular text and character references to represent Text nodes in the DOM.
When XHTML documents are serialized, prefixes and namespace declarations are adjusted as needed.