@Override
    public void startElement(String uri, String localName, String qName,
            Attributes atts) throws SAXException {
        if (!alreadyWarnedAboutRdf
                && "http://www.w3.org/1999/02/22-rdf-syntax-ns#" == uri) {
            warn("This validator does not validate RDF. RDF subtrees go unchecked.");
            alreadyWarnedAboutRdf = true;
        }
        if (!alreadyWarnedAboutOpenMath
                && "http://www.openmath.org/OpenMath" == uri) {
            warn("This validator does not validate OpenMath. OpenMath subtrees go unchecked.");
            alreadyWarnedAboutOpenMath = true;
        }
        if (!alreadyWarnedAboutInkscape
                && (("http://www.w3.org/2000/svg" == uri && attrsContainInkscape(atts))
                        || "http://www.inkscape.org/namespaces/inkscape" == uri || "http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd" == uri)) {
            warn("This validator does not validate Inkscape extensions properly. Inkscape-specific errors may go unnoticed.");
            alreadyWarnedAboutInkscape = true;
        }
        if (!alreadyWarnedAboutSvgVersion && "http://www.w3.org/2000/svg" == uri && hasUnsupportedVersion(atts)) {
            warn("Unsupported SVG version specified. This validator only supports SVG 1.1. The recommended way to suppress this warning is to remove the \u201Cversion\u201D attribute altogether.");
            alreadyWarnedAboutSvgVersion = true;
        }
    }
@see nu.validator.checker.Checker#startElement(java.lang.String,
     java.lang.String, java.lang.String, org.xml.sax.Attributes)
src/nu/validator/checker/UncheckedSubtreeWarner.java