As the other comment says, it's a valid URL but to really rub it in, the canonical link RFC (6596) says it's valid and then also uses a relative URL in an examp

As the other comment says, it's a valid URL but to really rub it in, the canonical link RFC (6596) says it's valid and then also uses a relative URL in an example like they were just waiting for someone to be wrong about this on the internet.