Currently, our fonts are:
font-family: Cambria Math, Latin Modern Math, STIX Math, LatinModernMathWOFF, MathJax_Main, STIXGeneral, serif;
The GUST people are currently working on fixing the ascent/descent bugs in their fonts. When this is ready, we should update our WOFF fonts and try to always use Latin Modern Math by default. This will give a default rendering consistent with the current PNG image rendering (computer modern style).
(Of course users could still easily rely on custom stylesheets or browser add-ons to change the default font-family)