I closed #7897 because the definition of <custom-ident> in the spec actually ensures that transition: ease-in ease-out expands with transition-timing-function: ease-in and transition-property: ease-out.

However, serializing a shorthand value requires ordering its longhand values in canonical order. And the canonical order of transition longhands puts transition-property before transition-timing-function.

So I think warning about placing transition-property after any other longhand when appropriate, would be helpful.

But how should transition serialize when, for example, all its longhands are declared with their initial value, but transition-property is declared with ease?

(1) with transition: ease ease?

(2) Always serializing all longhand values, as with many shorthands in mainstream browsers, is simple but goes against the shortest serialization principle.

(3) I do not known if replacing an unknown transition-property with unknown or not all (like for invalid <media-query>) or none (which requires accepting in <single-transition-property>), would break back-compat.

Note: both Chrome and FF serialize transition: ease for the above case, which does not round-trip, noting that Chrome puts the value of transition-property last, like animation-name in animation, which is convenient to disambiguate <custom-ident> when reading grammars from left to right.