Note the following two principles in CSSOM:

If component values can be omitted or replaced with a shorter representation without changing the meaning of the value, omit/replace them.

If either of the above syntactic translations would be less backwards-compatible, do not perform them.

Blink and Safari are the only browsers that support scroll-snap-type values like "x" and "x proximity", and they give "x proximity" as the computed value, so expecting "x proximity" would be more backwards-compatible.

@tabatkins please review

Blink and Safari are the only browsers that support scroll-snap-type values like "x" and "x proximity", and they give "x proximity" as the computed value, so expecting "x proximity" would be more backwards-compatible.

Back-compat is about pages depending on the behavior, not browsers happening to have done it a particular way in the past. (That's colloquially called "bugwards compat". ^_^) This is new enough that I doubt anyone's depending on our too-long serializations, so we should continue to follow shortest-serialization principle until shown evidence otherwise.