I probed a bit more into HTML output and the column width of tabulary
entries is always present in the HTML output. I don't yet know where
these relative widths are calculated, but I do roughly know how they are
calculated: from the width of table definition in the *.rst files. This
information should be sufficient to define a (relative) width instead of
the generic latex command "\begin{tabulary}{\textwidth}{|L|L|L|L|}".