...elements. Adds missing pseudo-element type passed into computed
properties getter.
Previously, due to this bug, we were using the element's computed
properties as the previous computed properties for its pseudo-elements.
This caused an excessive number of unintended CSS transitions to run.
The issue was particularly noticeable in Discord's emoji picker, where
each emoji has `::after` pseudo-element. We were incorrectly triggering
transitions on all their properties, resulting in significant
unnecessary work in style computation and animation event dispatching.
