I've used HttpUnit, and it's reasonably good. Now I'm trying HtmlUnit.
One advantage seems to be Xpath support. Looking at the javadocs, it appears that you can get the document node(s) indicated by an Xpath expression as
XPath path = new HtmlUnitXPath("a/b/c");
List results = path.selectNodes(domNode);From there it should be easy to make assertions about the matching node(s).