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In various places of the URL parser, the "missing pieces" of a relative are filled in from the base URL. For example, in relative state: > EOF code point > Set url's host to base's host, url's port to base's port, url's path to base's path, and url's query to base's query. The base URL’s username and password are always ignored. Is this deliberate?
Yes. Unless you have evidence to the contrary, of course.
For future archaeologists: https://github.com/whatwg/url/commit/06b2e6895dff2e6efaeba65f4eb1bc27ecc21581