SVG 1.1 SE depends on CSS2, with some weasel words about preferring the definitions in CSS 2.1 for those features which are in both CSS2 and CSS 2.1. It also depends on features in CSS2 which were removed from CSS 2.1, primarily the webfont material which is now in CSS3 Fonts.
While there are many changes between CSS2 and CSS 2.1, these almost all affect the CSS Box model and thus, have no impact on SVG. One change which does affect SVG is the specificity of @style, which is now higher than that of an id selector. (In CSS2, a rule with multiple ids in the selector would be more specific than a style attribute so would take precedence. In CSS 2.1 the style attribute always wins. Implementations all seems to assume @style wins anyway so this is not a breaking change. SVG2testsuite needs to test this).
In SVG2 we should clearly depend on CSS 2.1 which is now a Rec.
SVG 1.1 has vertical writing and bidi; we should reference CSS Writing Modes for that.
In SVG2 we should move from the CSS2 definition of webfonts to the CSS3 Fonts version (currently WD). There are some changes to existing functionality, and some useful new additions like support for OpenType features. SVG2 testsuite should have a port of the CSS3 Fonts testsuite. This may help it to exit CR.
SVG2 should depend on 2D (and 3D?) Transformation, Transitions, and Animations.
SVG2 should also look at referencing CSS3 Image Values.
See: CSS current work