Try turning on the “preserve user agent string” field in the advanced settings. Looks like the security for that site may be turned up pretty tight and it is probably blocking access based of the PTST/ string that gets added to the UA string (or it could be blocking access from the IP addresses).
Not a bug with WPT itself, the site is actively blocking access which is what 403’s usually are.
The keepua (preserve user agent string) seemed to fix it. What actually does it do? Would there be any benefit/issues with turning this on for all tests?
It prevents WPT from appending a PTST/ number to the UA string and uses the browser’s original string. Leaving the PTST addition makes it possible to filter out test traffic in your analytics so I usually wouldn’t recommend always disabling it.
If appending keepua=1 to the querystring is the same as selecting “preserve user agent string” then the test works through the UI but not via the querystring. I think there could be a bug.
Think this may be an issue to do with where the tests are running. Preserve User Agent works in London but not Ireland. Created a new thread for this now: