Classie.JS

First of all, I've never used classie.js, so I can't help you with any practical experience.

But I've looked at the code of the library and it seems sound.

Right now, there are two ways to manipulate classes in JavaScript. The old, somewhat messy, way is element.className. It returns a string with all the classes an element has. The classes are space-separated, so any class manipulation is basically string manipulation. You can match them through regular expression to test the presence of a class, you can add them and remove them by appending or replacing the classes in the string. This works in pretty much any old browser.

The new way of manipulating element classes is element.classList. It's fairly well supported (80.33% of the currently used browsers), and it's really becoming the best way to manipulate classes. You have access to methods that add, remove, toggle and check for the existence of classes.

Classie.js covers both of these ways. It tests for the second, newer method and uses it if it's available. If not, it does it the old fashioned way, manipulating the className string. It has all the checks you'd expect from such a library.

For instance, jQuery (based on the code in the master branch of their GitHub repo) still uses only the first (old) way of manipulating classes. Apart from some extra internal checks and ability to add several classes to an element at once, classie.js does what jQuery does.

When it comes to cross-browser compatibility, it should work in pretty much anything. Any problems that might arise would be a result of bad usage. I think you should feel pretty safe using it.