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.
CSSToXSLFO is a utility which can convert an XML document, together with a CSS2 style sheet, into an XSL-FO document, which can then be converted into PDF, PostScript, etc. with an XSL-FO-processor. It has special support for the XHTML vocabulary, because that is the most obvious language it would be used for. The tool has a number of page-related extensions. It also comes with an API in the form of an XML filter.
The OpenLaszlo platform allows developers to create applications with the rich user interface capabilities of desktop client software and the instantaneous no-download Web deployment of HTML.