I was just skimming through the W3C spec [w3.org].
"CSS2 supports media-specific style sheets so that authors may tailor the presentation of their documents to visual browsers, aural devices, printers, braille devices, handheld devices, etc.
This specification also supports content positioning, downloadable fonts, table layout, features for internationalization, automatic counters and numbering, and some properties related to user interface."
The primary focus is accessibility for assistive technologies. A visual browser wouldn't need to take on all of this. And after all, CSS 1 support is still a bit quirky!
I assume that second paragraph above describes where the meat of the matter lies for visual browsers.