We should support json response type on XHR request since fetching JSON is probably the most popular use case of XHR today. Blink added the support in https://chromium.googlesource.com/chromium/blink/+/3799efe521ff29595f7960078cfcc048c348d471 The patch is V8 specific but we can at least grab their test and C++-side code.
*** This bug has been marked as a duplicate of bug 73648 ***