The W3C Test Suite License is interesting. You use it if you want to make claims about your software like “passes all 𝑛 W3C tests.” It allows you to copy the tests, but you are not allowed to make any changes to them; they must remain byte-for-byte identical to the original.

Totally makes sense if you’re going to make compliance claims.

Alternatively, you may use the tests under the 3-clause BSD license, which allows you to modify them, but then, you can’t make compliance claims.