This looks like a good one to get familiar with how tests files are processed and executed.
I'd like to work on this. @annevk any pointers or hints where to start would be appreciated :)

@muodov reading https://web-platform-tests.org/writing-tests/testharness.html#tests-for-other-or-multiple-globals-any-js would go a long way. Adding // META: global=window,worker to existing .any.js tests and converting .html tests (if any) to .any.js tests would be most of the work change-wise, I think. And then verifying that the tests still function in these additional environments and there are no new failures. (And if there are, that's exciting, you'll have found bugs.)

One complication of this is that service workers implies https, and this breaks some tests with cross-scheme errors ("Mixed Content" and such).
I am going to try and add serviceworker global wherever I can, and then go and fix the new failures caused by Mixed Content one by one. I guess it will be similar to this pr

Ah yeah, that's right. Sorry for not giving a fair warning about that. I think it's also fine to leave any such tests alone for the moment.