This page show a Mathjax bug about text size in svg rendering, leading to text overriding mathjs expression (for this issue)
font-family and font-face are inherit, but text size detection is failing a bit, as you can see with this sample where mathjs override text, « : - » becomes ÷ because mathjs is displayed a little bit too far to the left (text block override mathjs block)
If you swap serif/sans-serif (sans-serif on body and serif on #small), mathjs is displayed too far to the right in serif case (too much space and no more overriding)
svg.mtextInheritFont: true seems ok if done before MathJax loading but not after, already in this issue (cf comments in source code, to reproduce it comment the config before loading and uncomment the one after loading)
LaTeX string, change it and rendering will occurs on blur
normal (as body : serif), MathJax svg embed in our svg