thanks, committed to tex4ht (modulo copyright stuff :). tex live is frozen right now; will leave open until i can commit it there.

Very strange bug. I added the option showframe to the geometry package in my Latex file to test something, and found that htlatex started creating HTML files with extension .html instead of .htm, even thought I did not change my makefile where I tell it to use .htm. here is a MWE

---------------------------
\documentclass[12pt]{book}%
\usepackage[letterpaper,showframe]{geometry}
\begin{document}
test
\end{document}
------------------------------

Compiling with the following, telling it to use "htm" for extension, not "html"

htlatex foo.tex "htm"

but it created foo.html instead, ignoring the request. Now removing the showframe option from the Latex file:

---------------------------
\documentclass[12pt]{book}%
\usepackage[letterpaper]{geometry}
\begin{document}
test
\end{document}
------------------------------

Using same command to compile, will now create foo.htm as expected.

This cause my build to fail, because it was looking for .htm file and not .html file.

Using TL 2015 tex4ht.c (2012-07-25-19:36 kpathsea)