The reason is that the HTML-files are all in Unicode but the wincmp3report.css is in ANSI. This combination is not handled well in Firefox (works in Internet-Explorer though).Warning: Unexpected end of file while searching for ',' or '{'. Ruleset ignored due to bad selector.
Source File: file:///D:/Temp/cmp/wincmp3report.css Line: 1
Maybe the bug lies in Firefox and not in Compare it! but I would say it would be logical for the files to be saved in the same way just to avoid this confusion.
So either the wincmp3report.css could be saved in Unicode also. Or the HTML-Files could be saved in UTF-8 instead of Unicode which would also work. The latter would save some space too.