[css-backgrounds-3] `border-image` serialization is inconsistent between browsers

[AtkinsSJ]

The interop view for border-image looks great, but the serialization tests cheat a bit. For example looking at this WPT test, you see that every case checks for multiple possible serializations, because the major browsers don't agree on what those should be.

For the sake of authors being able to make use of the serialization, and for new browser implementers, it'd be helpful to decide on a single format for serialization.

I know property serialization isn't usually specified explicitly, sticking with the general principles of being brief but unambiguous and in spec order. So I guess I'm just asking for people to agree on what that means here.