If omitting the universal selector in the serialization is possible, we
should do it so we obtain a shorter serialization (to match the behavior
asserted in cssom/serialize-namespaced-type-selectors.html).
For example, if someone writes *|*::before and there is no default
namespace, we should serialize to ::before; however, if there is a
default namespace, we should serialize to *|*::before.
(This is the test case "Universal selector in any namespace followed by
pseudo element).
This matches the behavior implemented by WebKit; that one case in
particular isn't implemented by Gecko, but other cases where the
universal selector should be elided are implemented by Gecko but were not
previously by Servo.
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