I'm really only familiar with ASP.NET, but do other frameworks not include support for partial page rendering? The ASP.NET `UpdatePanel` is simple and even has gracious degradation. (If client doesn't support for whatever reason, it will just revert back to doing full page POST and refresh.)

Of course, this is just wholesale replacing a DOM subtree with another. But there's also probably some point where just parsing the new subtree is faster than attempting to create and apply a diff.