web.xml schema (simplified)

  <xsd:complexType name="web-appType">

    <xsd:choice minOccurs="0" maxOccurs="unbounded">
      <xsd:group ref="j2ee:descriptionGroup"/>
      <xsd:element name="distributable" type="j2ee:emptyType"/>
      <xsd:element name="context-param" type="j2ee:param-valueType">
        <xsd:annotation>
          <xsd:documentation> 
            The context-param element contains the declaration
            of a web application's servlet context
            initialization parameters.  
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>

      <xsd:element name="filter" type="j2ee:filterType"/>
      <xsd:element name="filter-mapping" type="j2ee:filter-mappingType"/>
      <xsd:element name="listener" type="j2ee:listenerType"/>
      <xsd:element name="servlet" type="j2ee:servletType"/>
      <xsd:element name="servlet-mapping" type="j2ee:servlet-mappingType"/>
      <xsd:element name="session-config" type="j2ee:session-configType"/>
      <xsd:element name="mime-mapping" type="j2ee:mime-mappingType"/>
      <xsd:element name="welcome-file-list" type="j2ee:welcome-file-listType"/>
      <xsd:element name="error-page" type="j2ee:error-pageType"/>
      <xsd:element name="jsp-config" type="j2ee:jsp-configType"/>
      <xsd:element name="security-constraint" type="j2ee:security-constraintType"/>
      <xsd:element name="login-config" type="j2ee:login-configType"/>
      <xsd:element name="security-role" type="j2ee:security-roleType"/>
      <xsd:group ref="j2ee:jndiEnvironmentRefsGroup"/>
      <xsd:element name="message-destination" type="j2ee:message-destinationType"/>
      <xsd:element name="locale-encoding-mapping-list" type="j2ee:locale-encoding-mapping-listType"/>
    </xsd:choice>

    <xsd:attribute name="version" type="j2ee:web-app-versionType" use="required"/>
    <xsd:attribute name="id" type="xsd:ID"/>
  </xsd:complexType>