You also want to try a bogus MIME type, e.g., `Content-Type: bogus`. That cannot be parsed and is usually treated differently (often similar to omitted). I'm somewhat surprised "direct + nosniff + incorrect content-type" would still be different. Is that because something earlier in the stack is making a call about nosniff or some such? With regards to the web-platform-tests changes: I'm a little unsure about them as Chrome and Firefox currently pass these tests. Which if anything indicates there's a problem with the specification.
Bug 1704115 Comment 13 Edit History
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You also want to try a bogus MIME type, e.g., `Content-Type: bogus`. That cannot be parsed and is usually treated differently (often similar to omitted). I'm somewhat surprised "direct + nosniff + incorrect content-type" would still be different. Is that because something earlier in the stack is making a call about nosniff or some such? With regards to the web-platform-tests changes: I'm a little unsure about them as Chrome and Firefox currently pass these tests. Which if anything indicates there's a problem with the specification. (1704115 does seem like great progress, to be clear!)