You're completely correct that IE 4.0 - 6.0 was kicking Netscape's butt in terms of CSS support (not difficult as Netscape 4.x was truly awful). But not quite right that it was the best or only until 2003.

Opera could probably claim best CSS in the late 90s. And by 2003, Netscape 6.0 & 7.0 and Mozilla Suite 1.0 would have better CSS 2 support than IE 6.