ipython/ipython — pull request #5183 by Zaharid (2014-02-21 13:25:42+00:00)
Title: DOMWidgets can add classes before display.

They also keep track of the classes added and removed from python with a
_classes trait.

This solution doesn't touch the JS code, and possibly there is a better
way. Why does the _css property behave so differently from the css classes?