The overflow definition has:
Applies to: block containers [CSS2], flex containers [CSS3-FLEXBOX], grid containers [CSS3-GRID-LAYOUT]
However that predates clip.
Firefox has since ~forever supported clip (used to be -moz-hidden-unscrollable on inline boxes).
I don't feel too strongly either way, but should we consider making clip work on inline boxes?
cc @Loirooriol