I believe the correct DOCTYPE is (with the correct GWT version, e.g. “2.8.1” - check e.g. with “mvn dependency:list -DincludeGroupIds=com.google.gwt” in your project)
<!DOCTYPE module PUBLIC "-//Google Inc.//DTD Google Web Toolkit ${gwt.version}//EN" "http://gwtproject.org/doctype/${gwt.version}/gwt-module.dtd">
Thank you, Henri. That format eliminated the IDE warning.
I did not find com.google.gwt in the dependencies, but I did find com.vaadin.external.gwt and used that version number. In my case it was 2.7.0 (And I am using Vaadin version 7.7.7)
Update: The URL given above is almost correct. (Actually I think it was correct for the past year, but something may have recently changed on the gwtproject.org website.)
You can no longer omit the “www.” prefix. Under the hood, the website will return a 301 Moved Permanently, and redirect you to the fully prefixed address. So it prolly still works for validation purposes, but Eclipse can’t deal with the 301.