Some commands work . . . others don't.
I'm having a ton of stupid problems with latex for no apparent reason. I've tried two different browsers, and I've run Firefox in safety mode with every add-on disabled.
If you look at the first line in both the message field and the preview field you'll notice that it's sometimes accepting input, in that it wont give me an invalid equation tag, but not actually doing anything to the text.
You can use this website to insert LaTeX. Texify - Online LaTeX equation writer, generates math formulas from LaTeX expressions.