https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle

The spec currently says that the list of declarations that a CSSStyleDeclaration returned by getComputedStyle includes only "supported CSS properties", which means that it doesn't include custom properties. I think it makes sense to expose these through computed style objects.

Gecko does this, both via item() and getPropertyValue(). For item() (and the indexed getter) custom properties appear after all of the built-in properties, sorted by the order that they happen to cascade in (though that should probably become lexicographically sorted or something).