It wasn't impossible to write cross platform browser stuff in the late 1990s, when most corporations started this whole "We'll standardize on browser X" policy making, but it required a discipline that had most developers throwing their hands up in the air in disgust.
I had these arguments many times back then. It was laziness more than anything else. We were writing cross-platform web applications without problems at that time. We were trying to convince other developers to follow the same route, but their attitude was mainly "IE has 90%+ market share, why bother?" They didn't believe a time would come when proprietary IE code wouldn't work - even if other browsers caught on, they were expecting them to copy the IEisms. They certainly didn't believe that even later versions of Internet Explorer wouldn't support their crappy code.
- IE4+ was the most standard. Yes, really. Those versions had a relatively complete implementation of CSS.
Let's not overstate things. Netscape bet on JSSS and when the W3C selected CSS as the standard instead, they scrambled to fix Netscape 4 to convert from CSS to JSSS on the fly. So Netscape 4 was exceptionally bad at CSS. Internet Explorer 4 was merely very bad at CSS. Opera was ahead at that time. I don't think you can call IE4 "relatively complete" unless you only compare it to Netscape 4, which was unusually bad.