Reproduction: look at https://wpt.fyi/results/xhr/xmlhttprequest-timeout-aborted.html%3Faborted%20immediately%20after%20send()?label=master&label=experimental&aligned; Safaridriver reports an error of:

Could not parse requested URL 'http://web-platform.test:8000/xhr/xmlhttprequest-timeout-aborted.html?aborted immediately after send()'
Traceback (most recent call last):
  File "/Users/runner/runners/2.163.1/work/1/s/tools/wptrunner/wptrunner/executors/executorwebdriver.py", line 317, in run_func
    self.result = True, self.func(self.protocol, self.url, self.timeout)
  File "/Users/runner/runners/2.163.1/work/1/s/tools/wptrunner/wptrunner/executors/executorwebdriver.py", line 388, in do_testharness
    protocol.webdriver.url = url
  File "/Users/runner/runners/2.163.1/work/1/s/tools/webdriver/webdriver/client.py", line 19, in inner
    return func(self, *args, **kwargs)
  File "/Users/runner/runners/2.163.1/work/1/s/tools/webdriver/webdriver/client.py", line 528, in url
    return self.send_session_command("POST", "url", body)
  File "/Users/runner/runners/2.163.1/work/1/s/tools/webdriver/webdriver/client.py", line 515, in send_session_command
    return self.send_command(method, url, body)
  File "/Users/runner/runners/2.163.1/work/1/s/tools/webdriver/webdriver/client.py", line 479, in send_command
    raise err
InvalidArgumentException: invalid argument (400): Could not parse requested URL 'http://web-platform.test:8000/xhr/xmlhttprequest-timeout-aborted.html?aborted immediately after send()'

There are only 9 cases like this in WPT, but two of them are the idlharness results for html/dom, which are very rich grounds for testing:

https://wpt.fyi/results/html/dom/idlharness.https.html%3Finclude%3D(Document%7CWindow)?label=master&label=experimental&aligned
https://wpt.fyi/results/html/dom/idlharness.https.html%3Fexclude%3D(Document%7CWindow%7CHTML.*)?label=master&label=experimental&aligned