Comment 8 for bug 1006650

Here's a work around:

Print your webpage into a pdf printer (I use CutePdf, http://goo.gl/XKnU).
Then, convert through calibre.

I have not tested this method, so use at your own risk.
If you can simplify the web page (like Article mode on the kindle), then that would make the process much more effective.
Good luck!