I cannot find a reasonable video or website which explains and teaches MathML in detail.
MathML Fundamental
http://www.w3.org/TR/MathML3/chapter2.html
MathMl introduction
http://en.wikipedia.org/wiki/MathML
MathML with html5
http://www.tutorialspoint.com/html5/html5_mathml.htm
MathML browser test