From testing at http://d-theme.com/clearlooks. A W3C CSS validation produces the following errors for both CSS 2.1 and CSS 3.

URI : http://d-theme.com/sites/all/themes/clearlooks/style.css?s
185 #header .navigation .navigation-primary ul.links a:hover Property -moz-border-radius-topleft doesn't exist : 2px
186 #header .navigation .navigation-primary ul.links a:hover Property -moz-border-radius-topright doesn't exist : 2px
234 #container Property xclear doesn't exist : both
752 .comment Property -moz-border-radius doesn't exist : 3px
753 .comment Property -webkit-border-radius doesn't exist : 3px
937 ul.primary li a Value Error : background-color none is not a color value : none

The XHTML passes all the tests.