The purpose of MathJax is to make mathematical formulas easier to read, not to emphasize text parts of your posts. Use italic and bold formatting for that.
A similar question was asked on Chemistry Meta and I did an experiment with a screen reader. The 'nicely formatted text'
So, in the heap of my data it looked like
$$\text{Diamond in a coal mine}$$
But the diamond was edited to:
Diamond in a coal mine
led to the following being read heard by a visually impaired user who wants to read it:
So, in the heap of my data it looked like
Diamond in a coal mine
Diamond in a coal mine text
Diamond in a coal mine
But the diamond was edited to:
Diamond in a coal mine
That single 'nicely formatted' sentence would be heard three times (with a random word 'text' in between). I guess it's even worse if you put color into the equation.