From #222 (comment)

FWIW you really want to check arc::ptr_eq(old.get_${style_struct}(), new${style_struct}()) or so to avoid most of these comparisons. That's equivalent to what gecko does in CalcStyleDifference: https://searchfox.org/mozilla-central/rev/f6a806c38c459e0e0d797d264ca0e8ad46005105/layout/style/ComputedStyle.cpp#133