How to get MathJax render the \qed-command? Is there a way to use it? Maybe aligning the $\Box$ \Box under the current line?
The command that gives the tombstone is part of amsthm (or other theorem environment packages). The positioning of the tombstone is actually quite complicated, and I do not believe it is currently implemented in MathJax, nor do I think it will be in the near future.
Let me emphasize: MathJax is not, and was never intended to be a full featured dynamic LaTeX parser. It is a platform for displaying mathematics coded using TeX or MathML notation in an aesthetic way.
If you must have an Halmos symbol, you will have to kludge it yourself by inserting sufficiently much spacing (possibly with
\qquads inside dollar signs) and place $\Box$ (
\Box) or $\blacksquare$ (
But let me ask: what's wrong with just typing the three letters "q.e.d."?