I think this issue was fixed in the past. I get a correct result for the MWE with current TeX4ht.

I do not know if this was submitted before or not. I googled this and did not see anything.

When specifying a border line (vertical rule) between any 2 columns for a table (or longtable) as in \begin{tabular}{c|c}, tex4ht generates border lines around the table itself as well, as if one had written \begin{tabular}{|c|c|}

Here is a MWE

----------------------
\documentclass[12pt]{book}%
\usepackage{longtable}
\begin{document}
\begin{tabular}{c|ccc}
X
&
Y
&z&d
\end{tabular}
\end{document}
-----------------------

htlatex foo.tex

The same problem when using longtable. I noticed the inner border line is a little more thick than the outer border lines.

This of course does not happen with pdflatex

TL 2015