I think the Mac version of IE had something like 99% compliance on the first Acid test, before other browsers had robust CSS support. You could even find a copy of the Acid test in an easter egg in the browser.
It was a fantastic browser at the time, but didn't align with Microsoft's future interests and the development team was reassigned.
Nope. Just the box model bug that affected all the CSS 2 compliant browsers. ;)
It was indeed a pretty good browser for its time, though.
IE5:Mac didn't have support for a lot of things IE6 had that we now consider standard, like :hover for links, for..in, Array.prototype.splice, XmlHttpRequest, contentEditable...
They do have MS-Office ports for Mac, so that product was probably written with much better separation of concerns.
http://en.wikipedia.org/wiki/Internet_Explorer_for_UNIX
And before that, MicroSoft was best known for its BASIC implementation that wound up on a whole range of eight-bit microcomputers.
https://en.wikipedia.org/wiki/Microsoft_BASIC
The versions for 6502-based micros shared a codebase:
http://www.pagetable.com/?p=46
And, in some versions, they had an easter egg hidden in the constants used to implement SIN: