FYI: Templates only need to contain a single Element to be used as the Composite component markup - the extra CSS/markup in my example was to show development-preview of the component UI. This does not get in the way of CssResource bundles or any other GWT Resource mechanisms, it is purely a development-time feature.
You may find full source code and tags for each step of project creation here: https://github.com/lincolnthree/errai-ui-demo
Errai UI is part of the Errai framework at JBoss, and will be available in the upcoming Errai 2.1 release. You can build 2.1-SNAPSHOT yourself by checking out the errai code here: https://github.com/errai/errai/tree/2.1/, but it should not be necessary in order to run the demo.
Please enjoy, and tweet any feedback you may have to https://twitter.com/lincolnthree