These are specified in https://drafts.csswg.org/css-scroll-snap-1/#longhands
<rdar://problem/73661278>
Created attachment 418626 [details] Patch
Created attachment 418627 [details] Patch
Created attachment 418637 [details] Patch
Committed r272035: <https://trac.webkit.org/changeset/272035> All reviewed patches have been landed. Closing bug and clearing flags on attachment 418637 [details].