I did found a way to convert (even bad) html into well formed XML. I started to base this on the DOM loadHTML function. However during time several issues occurred and I optimized and added patches to correct side effects.

  function tryToXml($dom,$content) {
    if(!$content) return false;

    // xml well formed content can be loaded as xml node tree
    $fragment = $dom->createDocumentFragment();
    // wonderfull appendXML to add an XML string directly into the node tree!

    // aappendxml will fail on a xml declaration so manually skip this when occurred
    if( substr( $content,0, 5) == '<?xml' ) {
      $content = substr($content,strpos($content,'>')+1);
      if( strpos($content,'<') ) {
        $content = substr($content,strpos($content,'<'));
      }
    }

    // if appendXML is not working then use below htmlToXml() for nasty html correction
    if(!@$fragment->appendXML( $content )) {
      return $this->htmlToXml($dom,$content);
    }

    return $fragment;
  }



  // convert content into xml
  // dom is only needed to prepare the xml which will be returned
  function htmlToXml($dom, $content, $needEncoding=false, $bodyOnly=true) {

    // no xml when html is empty
    if(!$content) return false;

    // real content and possibly it needs encoding
    if( $needEncoding ) {
      // no need to convert character encoding as loadHTML will respect the content-type (only)
      $content =  '<meta http-equiv="Content-Type" content="text/html;charset='.$this->encoding.'">' . $content;
    }

    // return a dom from the content
    $domInject = new DOMDocument("1.0", "UTF-8");
    $domInject->preserveWhiteSpace = false;
    $domInject->formatOutput = true;

    // html type
    try {
      @$domInject->loadHTML( $content );
    } catch(Exception $e){
      // do nothing and continue as it's normal that warnings will occur on nasty HTML content
    }
        // to check encoding: echo $dom->encoding
        $this->reworkDom( $domInject );

    if( $bodyOnly ) {
      $fragment = $dom->createDocumentFragment();

      // retrieve nodes within /html/body
      foreach( $domInject->documentElement->childNodes as $elementLevel1 ) {
       if( $elementLevel1->nodeName == 'body' and $elementLevel1->nodeType == XML_ELEMENT_NODE ) {
         foreach( $elementLevel1->childNodes as $elementInject ) {
           $fragment->insertBefore( $dom->importNode($elementInject, true) );
         }
        }
      }
    } else {
      $fragment = $dom->importNode($domInject->documentElement, true);
    }

    return $fragment;
  }



    protected function reworkDom( $node, $level = 0 ) {

        // start with the first child node to iterate
        $nodeChild = $node->firstChild;

        while ( $nodeChild )  {
            $nodeNextChild = $nodeChild->nextSibling;

            switch ( $nodeChild->nodeType ) {
                case XML_ELEMENT_NODE:
                    // iterate through children element nodes
                    $this->reworkDom( $nodeChild, $level + 1);
                    break;
                case XML_TEXT_NODE:
                case XML_CDATA_SECTION_NODE:
                    // do nothing with text, cdata
                    break;
                case XML_COMMENT_NODE:
                    // ensure comments to remove - sign also follows the w3c guideline
                    $nodeChild->nodeValue = str_replace("-","_",$nodeChild->nodeValue);
                    break;
                case XML_DOCUMENT_TYPE_NODE:  // 10: needs to be removed
                case XML_PI_NODE: // 7: remove PI
                    $node->removeChild( $nodeChild );
                    $nodeChild = null; // make null to test later
                    break;
                case XML_DOCUMENT_NODE:
                    // should not appear as it's always the root, just to be complete
                    // however generate exception!
                case XML_HTML_DOCUMENT_NODE:
                    // should not appear as it's always the root, just to be complete
                    // however generate exception!
                default:
                    throw new exception("Engine: reworkDom type not declared [".$nodeChild->nodeType. "]");
            }
            $nodeChild = $nodeNextChild;
        } ;
    }

Now this also allows to add more html pieces into one XML which I needed to use myself. In general it can be used like this:

        $c='<p>test<font>two</p>';
    $dom=new DOMDocument('1.0', 'UTF-8');

$n=$dom->appendChild($dom->createElement('info')); // make a root element

if( $valueXml=tryToXml($dom,$c) ) {
  $n->appendChild($valueXml);
}
    echo '<pre/>'. htmlentities($dom->saveXml($n)). '</pre>';

In this example '<p>test<font>two</p>' will nicely be outputed in well formed XML as '<info><p>test<font>two</font></p></info>'. The info root tag is added as it will also allow to convert '<p>one</p><p>two</p>' which is not XML as it has not one root element. However if you html does for sure have one root element then the extra root <info> tag can be skipped.

With this I'm getting real nice XML out of unstructured and even corrupted HTML!

I hope it's a bit clear and might contribute to other people to use it.