For the most part, they aren't. The tree/graph/graphic packages for TeX are not supported by MathJax. From their site:
Keep in mind that MathJax is meant for typesetting math on the web. It only replicates the math functionality of LaTeX and not the text formatting capabilities. Any text formatting on the web should be done in HTML and CSS, not TeX.
I'm quite sure I've read a few posts on the MathJax mailing list that stated that tikz-like features are not among the goals of MathJax.
You could of course make your tableau in TeX, compile it, take a screenshot, and upload that. But this has a downside (besides being convoluted): it largely defeats the purpose of the editing feature.
On the other hand, there are initiatives like XyJax, which is a limited port of the
But even then, you'd have to get this ported package approved of by the SE team, which often proves to be a big hurdle for this type of request.
In conclusion, I'm afraid that you'll have to work around this limitation.
At least for expository purposes, the screenshot method is quite reasonable, if you're willing to make the effort.