Some more schema definitions for 5.0/5.1 contain validation errors (which will raise validation errors if building instances in Eclipse). The problem is that includes/imports don't point to the web locations.
The same issue arose for jboss-5.1.xsd: https://jira.jboss.org/jira/browse/JBMETA-200

jboss-web_5_1.xsd:

<xsd:import namespace="http://java.sun.com/xml/ns/javaee" schemaLocation="javaee_5.xsd"/>
<xsd:include schemaLocation="jboss-common_5_1.xsd"/>

Change to:
<xsd:import namespace="http://java.sun.com/xml/ns/javaee" schemaLocation="http://java.sun.com/xml/ns/javaee/javaee_5.xsd"/>
<xsd:include schemaLocation="http://www.jboss.org/j2ee/schema/jboss-common_5_1.xsd"/>

===================

jboss-ds_5_0.xsd:
<xsd:import namespace="http://java.sun.com/xml/ns/javaee" schemaLocation="javaee_5.xsd"/>
<xsd:include schemaLocation="jboss-common_5_1.xsd"/>

Change to:

<xsd:import namespace="http://java.sun.com/xml/ns/javaee" schemaLocation="http://java.sun.com/xml/ns/javaee/javaee_5.xsd"/>
<xsd:include schemaLocation="http://www.jboss.org/j2ee/schema/jboss-common_5_1.xsd"/>

===================

jboss-client_5_1.xsd
<xsd:import namespace="http://java.sun.com/xml/ns/javaee" schemaLocation="javaee_5.xsd"/>
<xsd:include schemaLocation="jboss-common_5_1.xsd"/>

Change to:

<xsd:import namespace="http://java.sun.com/xml/ns/javaee" schemaLocation="http://java.sun.com/xml/ns/javaee/javaee_5.xsd"/>
<xsd:include schemaLocation="http://www.jboss.org/j2ee/schema/jboss-common_5_1.xsd"/>