As I wrote in http://lists.whatwg.org/pipermail/whatwg-whatwg.org/2010-May/026449.html :

The "rules for parsing a legacy color value" in
http://www.whatwg.org/specs/web-apps/current-work/complete/common-microsyntaxes.html#rules-for-parsing-a-legacy-color-value
specify that CSS2 system colors should be accepted, and that they
should be converted to a simple color.

It seems like a substantial amount of work to get dynamic change
handling correct for this case, since system colors can change
dynamically when the user changes system appearance.  I'd really
like to avoid having to add dynamic change handling for this, and
I'd also like to avoid having to make system colors dynamic in CSS
but static in HTML.


(The current wording in the spec seems to mandate that they are
static, which seems like a bad idea in itself.)