MathML

Test:
Changing DOCTYPE html PUBLIC to DTD XHTML 1.1 plus MathML 2.0 still does not render the MathML correctly
$$




a
+
b


2

codes : 
      <math xmlns="&mathml;">
        <mrow>
         <msup>
          <mfenced>
          <mrow>
            <mi>a</mi>
            <mo>l;+</mo>
            <mi>b</mi>
          </mrow>
          </mfenced>
          <mn>2</mn>
         </msup>
       </mrow>
      </math>