On: The fastest math typesetting library for the web

[runarberg]

<mover> has a boolean accent attribute so I guess you can write:

    <mover accent="true">
      <mi>B</mi>
      <mo>→</mo>
    </mover>

[throwaway290 → runarberg]

So maybe that's a problem with a particular latex->mathml convertor, but the idea itself should work.