Align equations at equal sign

From https://groups.google.com/d/msg/mathjax-users/x_cKEMVL240/o2K547Bktx4J

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Is there a simple way to align equations at the equals sign? Its particularly useful when showing a derivation. There is some information here: http://www.w3.org/TR/MathML3/chapter3.html#type.group-align but it is not working for me and I was wondering if there is a MathJax specific write up.

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There are still a number of MathML3 table features that are missing from MathJax, and the and related are among them. For now, you will have to use directly. If you are working from TeX, there are the AMSmath environments like \begin{align}...\end{align} that have already been mentioned. If you are producing MathML some other way, you will need to generate elements like the following:

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mtable columnalign="right left" columnspacing="0.28em" displaystyle="true">
    <mtr>
      <mtd>
        <mi>x</mi>
      </mtd>
      <mtd>
        <mo>=</mo>
        <mi>a</mi>
      </mtd>
    </mtr>
    <mtr>
      <mtd>
        <mi>y</mi>
      </mtd>
      <mtd>
        <mo>=</mo>
        <mi>b</mi>
      </mtd>
    </mtr>
  </mtable>
</math>

Hope that helps.

Davide