I found that htmldoc does document this, and appears to have a fix-- I can set this option higher:

--browserwidth
[download]

The default browser width is 680 pixels which corresponds roughly to a 96 DPI display. Please note that your images and table sizes are equal to or smaller than the browser width, or your output will overlap or truncate in places.

update: I found out the current version, 1.8.20, doesn't support printing html that appears inside of form related tags, such as input and textarea

-mark