code: public boolean isValidXHTMLFragment (@Nullable final String sXHTMLFragment)
    {
      return StringHelper.hasNoText (sXHTMLFragment) || parseXHTMLFragment (sXHTMLFragment) != null;
    }
docs: Check if the given fragment is valid XHTML 1.1 mark-up. This method tries
  to parse the XHTML fragment, so it is potentially slow!
  
  @param sXHTMLFragment
  The XHTML fragment to parse. It is not checked, whether the value
  looks like HTML or not.
  @return <code>true</code> if the fragment is valid, <code>false</code>
  otherwise.