Validating Html

Question

I'm attempting to validate HTML from a dom Document Node.
However, I can't even get to that step, because I can't build a Validator correctly, from the W3C official DTD. Why does my SchemaFactory think W3C's DTD is invalid? (Special thanks to Eliott Rusty Harold of IBM, for the original structure of this code)
public static boolean isHtml(org.w3c.dom.Document doc) throws SAXException, IOException {
          // 1. Lookup a factory for the W3C XML Schema language
          SchemaFactory factory = SchemaFactory.newInstance(javax.xml.XMLConstants.W3C_XML_SCHEMA_NS_URI);
        // 2. Compile the schema.
         URL schemaLocation = new URL("http://www.w3.org/TR/html401/loose.dtd");
         // TODO this next line is dying on me! It throws this Exception:
         //     "org.xml.sax.SAXParseException: The markup in the document preceding the root element must be well-formed."
         Schema schema = factory.newSchema(schemaLocation);
         // 3. Get a validator from the schema.
         Validator validator = schema.newValidator();
         // 4. Parse the document you want to check.
         Source source = new javax.xml.transform.dom.DOMSource(doc);
         // 5. Check the document
         try {
            validator.validate(source);                                                                 // SAXException if the document is not valid HTML
                                                                                                           // IOExeption for whatever reason...
            return true;                                                                                // the parser was working, and the document is valid HTML
        } catch (SAXException ex) {
             return false;
}

Answer 1

That's a DTD, not a schema. So telling the parser it's a schema is sure to cause problems.