Why does htmlextender reject some html?

I attached a textbox to an htmlextender, and it works, but certain html tags are rejected.

For instance, I went into source view, and put the following equation:

<p style="text-align:center"><strong>P₁₂ = P₁ + P₂</strong></p>

Submitting the textbox (via a submit button in a form) give me text minus that section (the equation).

Is this a bug?

Thanks,