One editor in my group was looking for --From Bert Bos (w3c)
Use mine.
Source code is at: http://www.w3.org/2003/01/BB_htmldiff-0.3.tar.gz
(The compiler may give an error about a good html diff tool, so I asked
inside W3C who was using what.
Here are non-existing i18n directory, but
you can ignore that.)

If you don't mind losing NS4 (which doesn't understand DEL/INS), my
preferred invocation is as follows:

htmldiff -i -w '<del>' -x '</del>' -y '<ins>' -z '</ins>' a.html
b.html

Without the summary of all answers I got
(except that options it seems that html group is using one, but didn't find any
link compares case-sensitively and uses SPAN in
addition to it) DEL/INS.

Bert