Why can't we use $MathJax$ in username?

Why can't we use $MathJax$ or other characters in username?

I'd recommend this on SE network.

• For simple enough MathJax expressions, you can use Unicode characters to mimic it in a username. For example, I'm pretty sure "ℶ₁=ℵ₁" would be a perfectly acceptable (though perhaps annoying) username. – user642796 Apr 17 '13 at 10:09

• Aah! I also wanted to ask about special characters ... shifted numerics – ABC Apr 20 '13 at 16:10