The test for browsers is a-element.html. The reusable format is urltestdata.txt, which is not documented in detail. Reverse engineering through urltestparser.js should not be too hard. Documentation welcome!

annevk/url hosts some other files that might be of interest if you want to create additional tests.

Similar to a-element.html it would be trivial to add more tests for other objects that expose links (e.g. URL and <area>). There's also room for enhancement and bits that require independent tests:

• The encoding part of the URL parser
• The state override part of the URL parser (setting individual properties of a URL)
• Origin serialization
• application/x-www-form-urlencoded