reproducible with latest JDK9 b48 and looks like it's a regression of recent changes in XML parser.

reproducible using test from JDK-8043750 that uses XML with missing quote

<applet-desc main-class="HelloWorld" name="Hello World" height=200 width=400 progress-class="CustomProgress" />

stack trace of the exception is

Caused by: org.xml.sax.SAXParseException; lineNumber: 26; columnNumber: 66; Open quote is expected for attribute "{1}" associated with an element type "height".
at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAXParseException(ErrorHandlerWrapper.java:203)
at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.fatalError(ErrorHandlerWrapper.java:177)
at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(XMLErrorReporter.java:400)
at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(XMLErrorReporter.java:327)
at com.sun.org.apache.xerces.internal.impl.XMLScanner.reportFatalError(XMLScanner.java:1436)
at com.sun.org.apache.xerces.internal.impl.XMLScanner.scanAttributeValue(XMLScanner.java:828)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanAttribute(XMLDocumentFragmentScannerImpl.java:1548)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanStartElement(XMLDocumentFragmentScannerImpl.java:1320)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(XMLDocumentFragmentScannerImpl.java:2790)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:605)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:511)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:876)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:805)
at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:140)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1212)
at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:648)
at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl.parse(SAXParserImpl.java:332)
at com.sun.deploy.xml.XMLParser.parse(Unknown Source)