Is the Maxthon browser just plain old IE under the hood? Or is it written from the ground up?
I believe Maxthon uses IE's rendering engine, but with a greatly changed UI. So a mix of the two, IE is doing to actual web-page drawing, but the rest of it is most likely from the ground up.
EDIT: As @kmonsoor notes in the comments and his own answer, this has now changed.
Maxthon is built ground-up based on WebKit.
But, a nice feature of Maxthon is it can switch its core engine. Unfortunately, there are still some legacy webpages that cannot be rendered properly using WebKit. For compatibility purpose, Maxthon can switch to Trident (that is same of IE).
Though, Maxthon uses the terms "Ultra", and "Retro" to refer "WebKit", and "Trident" respectively.
For more, plz check the "Isn't Maxthon 3 just a shell for Internet Explorer?" section in this page