It depends on your needs. Even Webkit can be overkill if you just want to display plain-old HTML. In some of my applications I use wxHTML instead. It's a component from wxWidgets library (not sure if you can use it without wx though). It's lightweight, supports just basic HTML stuff (some CSS support might be available in the next version), and has a really nice wxHtmlEasyPrinting control which gives nice print preview and also prints very well. As for cross-platform, wxWidgets works on Windows, Linux, Mac, and some other systems, so that would surely fit your requirements.
I don't recommend going down the Gecko road. It is possible to integrate it, but it's much harder to do than KHTML/Webkit - simply because Gecko was not built with embedding in mind.
