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`](https://github.com/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`