MathJax's TEX input jax implements:
 Nearly all plain TEX mathmode macros
 But not stretchable glue or rules, no
lowlevel box commands, and no
\halign
 Some standard
LATEX
macros and environments
(\frac , \sqrt[]{} , \mathbb , etc.)
 AMS symbols and AMS math
environments
(gather , split ,
align , etc.)
 Some extensions (e.g.,
\unicode )
The complete list of macros supported by MathJax
