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http://dev.w3.org/csswg/cssom/#cssstyledeclaration [[ setProperty(DOMString property, DOMString value ]] IE10 and WebKit/Blink seem to have [TreatNullAs=EmptyString] for the value argument.
https://dvcs.w3.org/hg/csswg/rev/2929095f9309 This meant I could remove [TreatNullAs=EmptyString] from cssFloat et al with no change in behavior for cssFloat = null.