Syntax::sntxb: Expression cannot begin with "/sub".

ToExpression::esntx: Could not parse log
<sub>10</sub>(''x'') as input.

<math 
  xmlns='http://www.w3.org/1998/Math/MathML'>
  <semantics>
    <mi>$Failed</mi>
    <annotation-xml encoding='MathML-Content'>
      <ci>$Failed</ci>
    </annotation-xml>
  </semantics>
</math>