Vanilla 1.1.9 is a product of Lussumo. More Information: Documentation, Community Support.
http://faketestsite.stackexchange.com/ is a place where you can create a user and test out posts for MathOverflow and work out the bugs. You can also view source in posts that you like and copy (hopefully) fragments and modify them. For posts that are edited, you can click on the time of last edit to get to (links to) the source.
If it weren't against MathOverflow policy, I would post a question like "What sample TEX\Markdown has been useful to you on MathOverflow that you would like to share?" Many of the common questions as well as many of the less common questions might get resolved by a post and then later turned into a handy if somehwat limited reference. Or someone could assemble the answers provided on meta into something useful.
Gerhard "If Wishes Were Action Items" Paseman, 2011.11.25
I believe that MathJax supports the {matrix} environment actually.
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