Karma is not a testing framework, nor an assertion library.
Karma just launches an HTTP server, and generates the test runner HTML file you probably already know from your favourite testing framework.
So for testing purposes you can use pretty much anything you like. There are already plugins for most of the common testing frameworks:
Throughout the development of AngularJS, we've been using JSTD for
testing. I really think that JSTD is a great idea. Unfortunately, we
had many problems with JSTD, so we decided to write our own test
runner based on the same idea. We wanted a simple tool just for
use the awesome Socket.io library and Node.js.