1. The "hidden" element at the root of the IE DOM tree, only accessible with "*" via CSS
2. inline-block was broken on IE6 (but fixable with hacks), and no transparent PNG support.
3. The outline CS property was implemented the same as background in IE
Best of all was the dog slow IE 6 JavaScript interpreter, which was like 50x slower than Firefox and IE 7+.
TBH I found cross platform development during that time easier than when smart mobile phones came out. There was about 20-30 slightly incompatible forks of the Android Browser/Webkit and the processors were slow and everything was JS. Blackberry OS was still a thing. I also don't miss the problems with iPhone 3GS and 3D background layer flickering.
Also programming cross platform at the time showed me how to write lightening fast JS. That has made me a lot of money when people come to me to fix their crap web apps.
It was a DXImageTransform Alpha filter. Complete pain to use.
In the recesses of my brain somewhere is a memory that it was related to ActiveX, and/or the CSS filter property. You set some special filter value that invoked ActiveX? Maybe I'm remembering incorrectly but if I think about it too deeply I might start screaming and never stop.
I don’t miss those days.
Now you can "copy URL" or "copy URL parameters" but there's no way to copy them all in one go. You need to manually combine them in an external text editor which is annoying.