SAXParseException: Element "web-app" does not allow "sometag" here

Couple of days back, I was asked to look into some web server related issue, at our partner's site. According to them, they packaged and deployed the web application, per the instructions of Sun Java Web Server (aka iPlanet web server) documentation -- yet they couldn't access their application using a web browser. They gave me a clue that they noticed some error (see below) during the web server start up:

[23/Mar/2006:04:07:29] failure (11038): WEB4220: The web application [/mywebapp] is unavailable because of errors during startup. Please check the logs for errors

The first thing I did was to check the actual log (<webserver_root>/<server_instance>/logs/errors) file for a more detailed error message, and found the one that I'm looking for:

[23/Mar/2006:04:07:26] info (10896): WEB0100: Loading web module in virtual server [https-v490s001] at [/mywebapp]
[23/Mar/2006:04:07:26] info (10896): WEB0100: Loading web module in virtual server [https-v490s001] at [/search]
[23/Mar/2006:04:07:28] info (10896): CORE3282: stdout: PARSE error at line 27 column -1
[23/Mar/2006:04:07:28] info (10896): CORE3282: stdout: org.xml.sax.SAXParseException: Element "web-app" does not allow 
"mime-mapping" here.
[23/Mar/2006:04:07:28] failure (10896): ContextConfig[/mywebapp] WEB3524: Parse error in application web.xml

It clearly says that the problem is with the mime-mapping tag in mywebapp's web.xml file. The last few lines of web.xml are like this:

        ...
        ...
       <welcome-file-list>
                <welcome-file>default.jsp
        </welcome-file-list>
        <mime-mapping>
                <extension>xsd
                <mime-type>text/xml
        </mime-mapping>
</web-app>

The real problem is the actual order of welcome-file-list and mime-mapping tags in web.xml. mime-mapping tag should appear before welcome-file-list in web.xml file. So, swapping welcome-file-list and mime-mapping tags fixed the issue, and the web application is accessible through a web browser, now.

The key is finding the real order of all tags that we define in web.xml. All web.xml files should conform to the XML DTD for a Servlet web-app (war) module, in order for the web server to load the application, properly. The last known published DTD is available in Sun Microsystem's web site at: XML DTD for a Servlet 2.3 web-app (war) module. Apparently the following piece {in DTD} helped me resolving the issue:

<!--
The web-app element is the root of the deployment descriptor for
a web application.
-->
<!ELEMENT web-app (icon?, display-name?, description?, distributable?,
context-param*, filter*, filter-mapping*, listener*, servlet*,
servlet-mapping*, session-config?, mime-mapping*, welcome-file-list?,
error-page*, taglib*, resource-env-ref*, resource-ref*, security-constraint*,
login-config?, security-role*, env-entry*, ejb-ref*,  ejb-local-ref*)>

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