Yeah but TeX has problems on its own which makes it a poor choice as a native language. E.g. if you wanted to create an anchor inside your expression, or apply some DOM manipulations. Then there is the whole problem of TeX not having a clear specification, and being turing complete. Both issues make it a noop for browsers to implement as a markup language. Like if you wanted to dynamically change the color of the denominator in the expression 1/x. Which node would you target, and which attribute would you change, and to what, in your script if the expression markup looks like this:

    \frac{1}{x}

How about now:

    <mfrac>
      <mn>1</mn>
      <mi>x</mi>
    </mfrac>

MathML is a lot easier for library authors target and for web developers to work with, which makes it a superior choice as a native language to TeX.