The Language — Rules

SVG and MathML within block or inline XHTML element is send for validation against the defined schema.

...
<mode name="block_inline">
  <namespace ns="http://www.w3.org/2000/svg">
    <validate schema="http://www.w3.org/TR/2002/SVG.xsd"/>
  </namespace>
  <namespace ns="http://www.w3.org/1998/Math/MathML">
    <validate schema="http://www.w3.org/Math/XMLSchema/mathml2/mathml2.xsd"/>
  </namespace>
</mode>
...

The default rule is to reject namespace. Elements from undefined namespaces are thus treated as invalid by default.