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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.)
WFM: <http://www.whatwg.org/html/#rules-for-parsing-a-legacy-color-value>