Update 07/2020:
The code bellow is not needed anymore. TeX4ht has now full support for the MathJax output. The equivalent of the code bellow is this:
make4ht filename.tex "mathml,mathjax"
If you want to keep your LaTeX math as plain text, use just:
make4ht filename.tex "mathjax"
make4ht is replacement for htlatex that converts to HTML5 and UTF-8 by default, so no special configurations as in the old answer are necessary.
Original answer:
You can use TeX4ht. It can output math as mathml which can be then displayed with mathjax. You can also configure html header to load mathjax script from mathjax's cdn:
\Preamble{xhtml,mathml}
\Configure{VERSION}{}
\Configure{DOCTYPE}{\HCode{<!DOCTYPE html>\Hnewline}}
\Configure{HTML}{\HCode{<html>\Hnewline}}{\HCode{\Hnewline</html>}}
\Configure{@HEAD}{}
\Configure{@HEAD}{\HCode{<meta charset="UTF-8" />\Hnewline}}
\Configure{@HEAD}{\HCode{<meta name="generator" content="TeX4ht
(http://www.cse.ohio-state.edu/\string~gurari/TeX4ht/)" />\Hnewline}}
\Configure{@HEAD}{\HCode{<link
rel="stylesheet" type="text/css"
href="\expandafter\csname aa:CssFile\endcsname" />\Hnewline}}
\Configure{@HEAD}{\HCode{<script type="text/javascript"\Hnewline
src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"\Hnewline
></script>\Hnewline}}
\Configure{@HEAD}{\HCode{<style type="text/css">\Hnewline
.MathJax_MathML {text-indent: 0;}\Hnewline
</style>\Hnewline}}
\begin{document}
\EndPreamble
With this config file, we request to output math as mathml, then configure html header so resulting file is html5. Save it as for example ht5mjlatex.cfg and then call from command line:
htlatex filename.tex "ht5mjlatex.cfg, charset=utf-8" " -cunihtf -utf8"
or you can use William F. Hammond's script (at bottom of the page)