I have a demo site for my new PHP based Milonic Menu builder at demo.portalparts.com. I was just talking to a client that is running IE 5.1 on a MAC OS 9.2 and they do not see the Horizontal Menu but do see the vertical ones ok.
Happens on Macs with IE that are not using OS X. Microsoft might have fixed this in the latest version, but I doubt it. They've had a multipart form vulnerability since version 4. FYI: They, MS, don't support OS9 browsers anymore.
