When using Common HTML, bad formats display as _[Math processing error]_ for a moment then just disappear.  Here's an example:

> $\begin{notanenvironment}
\notacommnd
\end{notanenvironment}$

For me, on Chrome 45.0.2454.101 in Windows 10, HTML-CSS renders with the box as previously.

Note that when I jump to [this answer directly](http://meta.math.stackexchange.com/questions/21505/mathjax-2-6-beta/21535#21535), it remains as _[Math processing error]_ (in red). (And when editing this answer the preview remains in this state for seconds -- after rerendering for every keystroke -- then returns to blank as described above, if you wait.)