BOM can override the encoding when decoding the response in responseText.

On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".

PASS xhr.responseText is "三村かな子"
PASS xhr2.responseText is "三村かな子"
PASS xhr3.responseText is "三村かな子"
PASS successfullyParsed is true

TEST COMPLETE