The following snippet, when compiled as part of a LaTeX document with appropriate packages loaded, would produce a numbered (well, \tagged) equation:
\begin{equation}
\tag{1}
\begin{split}
4 & = 2\cdot2 \\ & = (1 + 1)(1 + 1) \\ & = 1 + 1 + 1 + 1 + 1.
\end{split}
\end{equation}
(In case you think, as I briefly suspected, that it's just the numbering automatically provided by {equation}, it doesn't seem to be; it works just as well if you use \tag{I am not a number} instead of \tag{1}.) However, under MathJax, it does not.
\begin{equation} \tag{1} \begin{split} 4 & = 2\cdot2 \\ & = (1 + 1)(1 + 1) \\ & = 1 + 1 + 1 + 1 + 1. \end{split} \end{equation}
The same thing seems to happen if the inner environment is, say, {aligned}. Is this a bug?
