I believe this is WAI: https://drafts.csswg.org/css-pseudo-4/#generated-content

When their computed content value is not none, these pseudo-elements generate boxes as if they were immediate children of their originating element, and can be styled exactly like any normal document-sourced element in the document tree.

This implies that if the computed content value *IS* none, they cannot be styled, and therefore cannot have animations attached to them.