IE 5.5 on OS 9 was, at the time, the best browser available for the platform. Literally every fruity iMac had it and used it by default that I ever encountered. Including my own.

It was the best browser on any platform at the time, and paved the way for strong web standards support.

People often forget about this when bashing IE.

IE was available for OSX all the way through 10.6 IIRC.

Was that native support, or was it based on OS 9 / Classic emulation? I know that the PowerPC versions of OS X (i.e. every OS X up to and including 10.5) supported running OS 9 software in a sort of emulation layer (though I don't remember if this was first-party or third-party; I do recall PowerPC Macs with OS X having a "Classic Mode" that booted into OS 9, however).

Carbon, so not Classic, but based on the updated Classic Mac OS layer.

It was PPC only, last updated in 2003.