[css-transitions] `transition-property` needs to specify case sensitiveness

[Loirooriol]

https://drafts.csswg.org/css-transitions-1/#transition-property-property defines the grammar of transition-property as

none | [ all | <custom-ident> ]#

And https://drafts.csswg.org/css-values-4/#identifier-value defines <custom-ident> to be "fully case-sensitive".

However, standard properties (excluding custom properties) are ASCII case-insensitive, so it's not much clear how the comparison should be performed.

From the above I think I would expect:

• Custom idents are kept as-is, no casing normalization
• When checking if a property is in the list, comparisons are done case-insensitively if the property is case-insensitive, or case-sensitively if the property is case-sensitive.

But implementations normalize custom idents to lowercase when there is a case-insensitive match with a property that is case-insensitive:

document.body.style.transitionProperty = "lOreMipsUm, cOloR, --Foo, --foO";
document.body.style.transitionProperty; // "lOreMipsUm, color, --Foo, --foO"