IE 5.5 for OS X was by far the most standard compliant browser of its time. It supported more CSS than either NN 4.x or IE 5 for Windows. Nothing came to surpass it until Mozilla 1.0 and even then it wasn't a slam dunk.
Was gonna say IE5 on OS X was the opposite of “a dog of a browser”. It was the gold standard against which every other browser was compared because it was by far the most standards compliant browser of its day.
Also a quick correction, there was no IE5.5 for OSX. That was for Windows and used a diff rendering engine.
