java.sun.com is down and redirects to Java-related section of Oracle's website. Other than that all the DTDs and XSDs are in usual location, so there is no need for panic.

For example the mentioned http://java.sun.com/dtd/properties.dtd can be found with no issues. Other example is http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd that sits in usual location and I don't expect anyone was going to touch it.