This file fails to build with tex4ht only. Builds ok with pdflatex.

%--------------------------------
\documentclass{article}
\ifdefined\HCode
\def\pgfsysdriver{pgfsys-tex4ht.def}
\fi
\usepackage{tikz,graphicx}
%
\usetikzlibrary{trees}
\begin{document}
\begin{tikzpicture}
\node {root}
child {node {$\frac{a}{b}$}};
\end{tikzpicture}
\end{document}
%----------------------------

Using texlive 2014:

>latex --version

pdfTeX 3.14159265-2.6-1.40.15 (TeX Live 2014)
kpathsea version 6.2.0

htlatex foo.tex

gives

(/usr/local/texlive/2014/texmf-dist/tex/generic/pgf/modules/pgfmoduleplot.code.
tex)
(/usr/local/texlive/2014/texmf-dist/tex/latex/pgf/compatibility/pgfcomp-version
-0-65.sty
! Undefined control sequence.
\pgfsys@svg@newline ->\Hnewline

l.190 \pgfusepathqfill}

?

Build OK with texlive 2013

reference:
http://tex.stackexchange.com/questions/185349/error-using-pgfsysdriver-with-tex4ht-only-shows-up-with-texlive-2014-ok-with-t