[css3-transitions][css3-animations] should negative 'animation-durtaion' and 'transition-duration' be parse errors

http://dev.w3.org/csswg/css3-transitions/#the-transition-duration-property-
http://dev.w3.org/csswg/css3-animations/#the-animation-duration-property-

These sections of the specifications say that negative values of
transition-duration and animation-duration are treated as '0s'.
This is somewhat unusual; normally out-of-range values in CSS are
treated as parse errors.  Should we change these to match the normal
behavior?

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                           http://www.mozilla.org/   𝄂