[kibibu]

There were differences between IE on Windows, IE for Mac (which had an entirely different engine), Netscape, Gecko, and Safari.

If you needed to work around them, you could either sniff user agents, or abuse parsing inconsistencies to hide rules from each.

[JohnTHaller]

I used the parser inconsistencies myself. I created the first web app template that worked in IE 3/4 and Netscape 3/4 for a Fortune 500 back in the day.