private void allowAttribute() {
        if (!inOpenStartTag) {
            throw new RuntimeException("Called writeAttribute() while not in an open start tag");
        }
    }
Writes an attribute by XML.xmlEscape(value.toString(),false)

@param name the name of the attribute. An exception is thrown if this is null
@param value the value of the attribute. The empty string if the attribute is null or empty
vespajlib/src/main/java/com/yahoo/text/XMLWriter.java