Just an update (maybe pass this on to Itch, too):
If I edit the "index.html" to replace the "*.gz" files with the un-gzipped files (and actually un-gzip the files within the Itch folder for the game), everything seems to work okay.
My guess is that since Itch is feeding the files locally instead of a "real" web server, it is not sending the encoding and/or content-type headers (or any other web server headers, for that matter...)
(Oh, and just F.Y.I., I am a Linux user, not a Windows PC user, which is why I prefer the Itch app. I also do not always have Internet access to play online.)