@Override
    public void startDocument() throws SAXException {
        if (!documentStarted) {
            documentStarted = true;
            super.startDocument();
            startPrefixMapping("", XHTML);
        }
    }
Starts an XHTML document by setting up the namespace mappings
when called for the first time.
The standard XHTML prefix is generated lazily when the first
element is started.
tika-core/src/main/java/org/apache/tika/sax/XHTMLContentHandler.java