Lobo [download] is an open source web browser that is written completely in Java.
Lobo is being actively developed with the aim to fully support HTML 4, Javascript and CSS2. Lobo also supports direct JavaFX rendering.
The general goal of the Lobo browser effort is to produce a browser that is fast, complete, easy to extend, feature-rich and secure.
Flying Saucer is an XHTML renderer written in Java. It's 100% Java, not a native wrapper, and it only handles well-formed XHTML + CSS. It is intended for embedding web-based user interfaces into Java applications (ex. web photo album generator, help viewer, iTunes Music Store clone). It cannot be used as a general purpose web browser since it does not support the malformed legacy HTML found on the web, though recent work on compatibility libraries may be enough to do what you need. You may be able to work with legacy HTML (e.g. HTML that is not well-formed XML) by using a pre-processor that cleans it up; there are several of these, including JTidy and TagSoup.