[css-anchor-position] Clarify computed value serialization for anchor-scope

[tuankiet65]

anchor-scope takes the following value:

none | all | <dashed-ident>#

If the value is a list of <dashed-ident>, it's not clear if the computed value serialization should preserve the order as in the original list.

For example, if the original value is:

anchor-scope: --a, --b, --c

is this an acceptable computed value serialization:

anchor-scope: --b, --a, --c

[fantasai]

The current spec says the Computed Value is "as specified", so I think that would imply preserving the order. But whether it should or not is a legit question, since the behavior doesn't really care about order.

From a computed value perspective, we shouldn't care, and both of those should be treated as equivalent. From a serialization perspective, though, we need to define an order, and specified order is convenient because it doesn't defining require a sorting algorithm...