If you look at http://w3c-test.org/XMLHttpRequest/setrequestheader-case-insensitive.htm it fails in all of Safari, Chrome, Firefox, and Edge. The test is doing this:

    var client = new XMLHttpRequest()
    client.open("POST", "resources/inspect-headers.py?filter_value=t1,t2,t3", false)
    client.setRequestHeader("x-test", "t1")
    client.setRequestHeader("X-TEST", "t2")
    client.setRequestHeader("X-teST", "t3")
    client.send(null)
    assert_equals(client.responseText, "x-test,")

which asserts that the value of the "x-test" header that was sent to the server is "t1,t2,t3". In Safari, Chrome, Firefox, and Edge the value is "t1, t2, t3" (with spaces after the commas).

This sort of thing has come up before (e.g. whatwg/fetch#422)...

@annevk I don't know whether you want to fix this in XHR or in fetch or whether you think all browsers should change here or what...