Arn't they using the local IE engine?
That's exactly what I said. They use the COM control, that ships with IE and is installed on every windows PC and embed it in their application.
Drawback of this are: the control is slow, has no real API, has a restricted feature set and it behaves differently between different IE versions.
Webkit would have been my guess. Worst issue is that they are using IE 6.0 and not 7.0. :/
Read above. They use the control, which has whatever version the user has updated his system to. So for some users it's 6, for others it's 7 or even 8.
I'd go for WebKit simply because it's so amazingly compliant with all standards (first engine to pass the Acid3 test flawlessly with development builds!).
Opera and
Webkit achieved this at pretty much the same time. I'm using Opera 10.0 alpha and it renders Acid3 flawlessly and is much faster than Webkit.
Of course I know how Opera's licensing works, that's why I said they should use one of the open source engines.