I believe that isn't a problem to use HTML5 since you use deferred binding to detect if it's possible to do what you want in that given browser, in case it's not you may do something else that looks like what you want (a poor alternative) or a warning to your user.
Search about the Deferred Binding feature of GWT and about create components, certainly this project will be very cool, post here or send me a private message when you research/do more about it.
And about the eclipse plugin, he doesn't really matter what "kind" of HTML is, at the maximum you'll not able to use auto-completion on html5 components, but it's eval as plain text. Your only concern will be about the GWT and deferred binding, i believe in it's default implementation will not allow you to render a html5 component, but if you create your own component and rules of deferred binding it may be possible.
Hope it help, feel free to ask more question in here or by private message.
Feel free to ask me anything!
www.BlackBeltFactory.com/ui#!/ref=jmotta, SCJP 6, OCWCD JEE5, OCE EJB JEE6
Joined: Apr 15, 2010
Thanks a lot, Jayr - you've helped me a lot!
Now I've got an idea about "Deferred Binding"