Doctor Jimmy
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Most of the others are just one of the engines with a different shell slapped over it, really.
Not really. Opera is the only browser that uses Presto. And there are like 5 different engines on that list. A shell is a program that provides a different interface for an engine that is already installed. Maxthon is a shell because it uses the rendering engine that's already installed on the computer, and it requires IE to run. Chrome isn't a shell, because although it uses the same engine as Safari, it doesn't depend on Safari to run. Same with K-Meleon, Seamonkey, and Epiphany; they use Gecko, but do not depend on Firefox.