This is XHTML. It uses the XHTML namespace. The head tag is <html xmlns="http://www.w3.org/1999/xhtml">
The following is MathML. It uses the MathML namespace. The math tag is <math xmlns="http://www.w3.org/1998/Math/MathML">
You can look at the source of this page to see these.

E = m c 2

Now we are out of the   math   container. This is XHTML again. It uses the XHTML namespace.

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