Lobo is aiming for html 4 compliance.
http://lobobrowser.org/java-browser.jsp
One could argue that the rendering aspect of a browser is one of the most, if not the most, challenging aspects of developing a browser. (Networking, thread management, security sandbox, etc. are all pretty much available out of the box on the JVM ...)
So, if you're not up to writing your own components for dealing with correctly rendering documents based on xhtml, css, and javascript (not to mention various common plugins such as flash), then why not consider contributing to an open source effort such as lobo?