DocumentModeAsserter is asserting that the Document.compatMode does not equal "BackCompat", which corresponds to quirks mode. You should be able to use any variation of the strict mode doctype and still pass the asserter, as long as the Document.compatMode is CSS1Compat. If your app is in quirks mode, we tell you to use <!DOCTYPE html>, but the strict doctype would work as well.
Also, you can disable the warning by extending the allowed doctypes property to include BackCompat. http://gwt-code-reviews.appspot.com/1422816/ -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
