[l2h] Math formula background color?

[Philipp K. Janert, Ph.D.]

At the end of this message is a trivial latex file.
It contains three display formulas, all essentially
the same, except for the specific way the absolute
value is designated: once simply using |...|, once
using \left|...\right|, and once using \bigl|...\bigr|.

The file compiles without problem under pdflatex.
I also compiles under latex2html  and images are 
generated for all three formulas. However, the last
one (and only the last one) shows up on a gray 
background.

Adding a 
\usepackage{amsmath}
to the preamble makes this problem go away, and
all generated images are now on white (transparent)
background.

I wonder why this is? 

I could understand that something would fail entirely
to compile if a required package (such as amsmat)
was not loaded - but everything compiles fine, no
matter whether amsmath is loaded or not. But why
would the absence of a required package change 
the background color for a formula image, which is
otherwise generated correctly?

Best regards,

			Ph.

>>> File starts below >>>

\documentclass{article}
\usepackage{html}

\begin{document}

Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Sed
interdum. Morbi tu rpis. Aenean a massa ac velit laoreet
\[
| 1 + 2 |
\]

eleifend. Curabitur velit. Cras justo libe ro, porttitor ac, sodales
eu, pharetra eget, felis. Nunc eu tortor. Aenean vulpu tate orci at
\[
\left| 3 + 4 \right|
\]

sapien. Etiam tortor lectus, viverra sed, pellentesque vel, vestibu
lum non, sem. Sed vitae metus id sem scelerisque posuere. Sed ornare,
\[
\bigl| 2 + ( 3 + 4 ) + 5 \bigr|
\]

pede et se mper feugiat, massa leo tempus wisi, in auctor enim orci ac
orci. Suspendisse po tenti. Ut pulvinar tempor lectus. Pellentesque

\end{document}