Bug 1385139 comment 9 has some background but basically in bug 1260655 we decided that in order to get the right overlapping behavior when creating Keyframe objects from @keyframes, we should expand shorthands into longhands. Some of the difficulties are described in [1]. This: * is not very helpful for authors (information lost; can't use getKeyframes() to setup another animation with equivalent behavior) * means we need to expand variable references in order to map them to longhands * introduces various complications (e.g. if we expand variables at the last minute we need carry context around as is impossible if the effect has no target; if we expand variables early on we need to re-create the keyframes every time variables anywhere change, etc.) We should revisit that initial decision to expand shorthands and see just how complex it would be to preserve the original values. I think there may be complications with logical properties too that necessitate this. [1] http://searchfox.org/mozilla-central/rev/e8c36327cd8c9432c69e5e1383156a74330f11f2/layout/style/nsAnimationManager.cpp#635-677