HTTP/2 response contains a content-type header and HTTP/3 response does not. so in HTTP/3 case it does not. BBrowser implement sniffingo f types by looking at up to 521bytes (or untill the end of stream) to figure out the type. Firefox is waiting for 512bytes or the end of the steam and this is what is blocking the promise. II tried on Chrome, but I could not make it use HTTP/3 and HTTP/2 response contains the header the same as with Firefox. Does it work for you with Chrome when HTTP/3 is used?
Bug 1793342 Comment 2 Edit History
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HTTP/2 response contains a content-type header and HTTP/3 response does not. so in HTTP/3 case, it does not. Browser implements sniffing of types by looking at up to 521bytes (or until the end of the stream) to figure out the type. Firefox is waiting for 512bytes or the end of the stream and this is what is blocking the promise. I tried on Chrome, but I could not make it use HTTP/3 and HTTP/2 response contains the header the same as with Firefox. Does it work for you with Chrome when HTTP/3 is used?