When developing RIA, AJAX and other xHTML web applications, multi-tier or otherwise, it pays to be cognizant of browser compatibility.  Since the beginning we always have to be mindful of which browser and what version, duplicating effort by creating multiple versions of content and Javascript.

None of that has changed with HTML 5 so here is a link to a pretty decent set of browser compatibility resources.  Click HTML5 Tests to check your browser.

http://www.html5rocks.com/en/resources.html

I am growing fond of GWT (>2.1); I like the approach of writing Java classes for client/browser-side JVM deployment vs. plain Javascript.  GWT leverages this class-based development approach.  This is similar to using JApplet in browsers with recent versions of Java 1.6.  JApplet lets you embed a rich Swing application in a browser frame, but only for late JRE 1.6 versions.