According to https://www.w3.org/TR/REC-xml/#sec-prolog-dtd, XML documents SHOULD begin with an XML declaration (e.g. <?xml version="1.0" encoding="UTF-8" ?>. Although this isn't mandatory, some things may not recognized the output as an XML document. For example, the 'file' command says the data is HTML. This isn't a big deal, but is technically more correct.