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http://xhr.spec.whatwg.org/#response-entity-body-0 [[ The response MIME type is the MIME type the Content-Type header contains excluding any parameters and converted to ASCII lowercase, or null if the response header can not be parsed or was omitted. ]] This needs to become much better defined.
Fetch has a hook of sorts for this now. Still not very well defined.
https://github.com/whatwg/xhr/pull/131