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From https://groups.google.com/d/msg/mathjax-users/4btEfmThia0/GqH99mEZGLwJ
MathJax doesn't implement tabular, because it is really a text-mode environment. You can use the array environment as a replacement. E.g.
\newcommand\T{\Rule{0pt}{1em}{.3em}} \begin{array}{|c|c|} \hline X & P(X = i) \T \\\hline 1 \T & 1/6 \\\hline 2 \T & 1/6 \\\hline 3 \T & 1/6 \\\hline 4 \T & 1/6 \\\hline 5 \T & 1/6 \\\hline 6 \T & 1/6 \\\hline \end{array}
should get what you are looking for.
Davide