The function transformToXML has a problem with the meta content type tag. It outputs it like this:
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
which is not correct X(HT)ML, because it closes with '>' instead of with '/>'.
A way to get the output correct is to use instead of transformToXML first transformToDoc anf then saveHTML:
$domTranObj = $xslProcessor->transformToDoc($domXmlObj);
$domHtmlText = $domTranObj->saveHTML();