Detailed are here:
http://tex.stackexchange.com/questions/220204/problem-with-titlesec-package-with-tex4ht-when-using-starred-version-of-section

summary:
When compiling this

\documentclass[12pt,notitlepage]{report}
\usepackage{amsmath}
\usepackage{titlesec}
\begin{document}
\section*{$ \left( a \right) $}
\end{document}

using htlatex foo.tex

the error is

(./foo2.aux)
! Argument of \im:g has an extra }.
<inserted text>
\par
l.6 \section*{$ \left( a \right) $}

The problem goes away when changing \section* to \section or when removing the titlesec package

>pdflatex foo2.tex

This is pdfTeX, Version 3.14159265-2.6-1.40.15 (TeX Live 2014)

File List
report.cls 2014/09/29 v1.4h Standard LaTeX document class
size12.clo 2014/09/29 v1.4h Standard LaTeX file (size option)
amsmath.sty 2013/01/14 v2.14 AMS math features
amstext.sty 2000/06/29 v2.01
amsgen.sty 1999/11/30 v2.0
amsbsy.sty 1999/11/29 v1.2d
amsopn.sty 1999/12/14 v2.01 operator names
titlesec.sty 2011/12/15 v2.10.0 Sectioning titles

This problem shows up when using images for math, not mathml.

workaround: do not use titlesec package with tex4ht when compiling math as images.