Where can I find valid values ​​for the version attribute of the web-app element in web.xml?

A typical web.xml is as follows:

<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
        "http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">

<web-app version="2.4">
  <display-name>Archetype Created Web Application</display-name>
</web-app>

How do I know which values ​​are valid for an attribute version <web-app>?

<web-app version="?">

Is it 2.5 or maybe 3.0?

Does anyone know the url using a schema or DTD to validate web.xml? The URL for DTDs of different versions of web applications is different, is there some kind of source where I can see the full list?

//doctype for servlet 2.3
<!DOCTYPE web-app
    PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
    "http://java.sun.com/dtd/web-app_2_3.dtd">

//doctype for servlet 2.2
<!DOCTYPE web-app PUBLIC
  "-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN"
  "http://java.sun.com/j2ee/dtds/web-app_2_2.dtd">