I'm mildly worried about printing error-level logging for things that might not be errors. In particular if you have a test that needs a 404 response, putting an error message in the log isn't that helpful. It may also cause problems in Gecko's CI where we try to parse out the actionable lines from the log; people get annoyed if that summary contains a lot of noise unrelated to actual failures.

I'd suggest maybe logging messages from 500 status at error level and picking a lower level like info for non-500 errors.

FYI: wpt-pr-bot didn't pick this up as a WebKit export because (I believe) the WebKit side doesn't have a See Also. So it won't auto-approve or auto-merge this even after the WebKit side is R+ or CQ+

+1 to @jgraham usually a web server doesn't log 404 as errors (this is the default in Apache and Nginx, as well as the current behaviour of wptserve for other handlers); the other changes look good.

Also this is linked to the wrong WebKit issue; should be https://bugs.webkit.org/show_bug.cgi?id=215829 / https://trac.webkit.org/changeset/266816/webkit

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