I have special case in my hand; currently I maintain a custom branch
of WebKit, I already have
implemented a module which sits closely with WebCore and it works just
fine. One of my
goals is to deliver these changes back to community and even may be
back to WebKit repository. In near
future I will be implementing at least 2-3 such modules, which work
closely with WebCore and
JavaScriptCore but they may not have to anything to do HTML browser
engine as such.