According to Bug 1341372 Comment 59, there are cases that property is animtable but the before_change and after_change are not able to be interpolable, e.g. 'auto' to something. But unfortunately does_animate()  does not check it at all so far.  http://searchfox.org/mozilla-central/rev/2fc8c8d483d9ec9fd0ec319c6c53807f7fa8e8a2/servo/components/style/properties/helpers/animated_properties.mako.rs#202-212
Note that as of changeset 4c45c167a91b7c6554d5a3cfd81aa74c46937ac5 we don't call `does_animate` anymore. The fact that needs_transitions_update_per_property does not check for interpolability still exists, however. As far as I can tell, this is just a minor optimization though, right? http://searchfox.org/mozilla-central/diff/4c45c167a91b7c6554d5a3cfd81aa74c46937ac5/servo/components/style/gecko/wrapper.rs#1329
Yes, right, this is a kind of optimizations.