To prevent that warning, you should add
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
to your “index.html”. The standard page for a GWT project in Eclipse doesn’t contain a DTD reference, so you should add “loose.dtd”. An alternative could be, that you add
<extend-configuration-property name="document.compatMode" value="BackCompat">
to your module XML file. But this only hides the warning and doesn’t solve the problem.