Mixing MathML markups and other markups to get special effects

A fraction with the animated throbber as denominator...
View the source...
$$\genfrac{}{}{0.3ex}{}{a}{}$$
See the difference between <mi> an <html:i>
(which doesn't understand the scriptlevel) :
$$\frac{a}{a}\frac{a}{a}$$
And there could be more...