CONSOLE WARNING: line 24: Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check https://xhr.spec.whatwg.org/.
Test XMLHttpRequest and document.xmlEncoding interaction.

The request body should contain the word "Проверка".

responseXML.xmlEncoding: windows-1251
responseXML.xmlVersion: 1.0
responseXML.xmlStandalone: true
Request body: <?xml version="1.0" encoding="windows-1251" standalone="yes"?><doc>Проверка</doc>