This has nothing to do with CSS3.

Wikipedia still looks good because it uses tables for layout. It's worthy of IE 5.5's good graces at that point.

Also probably because Google and Wikipedia's design wen't largely unchanged in the past 13 years.

True. And I think they went with the bare minimum necessary to display a functional web page (Google's extra junk is mostly analytics); the side-effect being a more usable page on older browsers.