Step
Test

Embedded testing can help your library stay stable and not in the ways you expect. Step Test is designed to be small and deliverabe with libs for all the things you didn't expect your lib consumers to do.

Over Riding Your Lib

For instance

1. You create a class called Car
2. That class has a function called drive.
3. Drive triggers an observable event.
4. Lib consumer overrides the function without knowing about the observable event.
5. Hooks things into the Car on "drive" event and every time the function is called nothing happens

This is where step test comes in, because you wrote tests and deliver them, the developer can run them manually, or they can be set to run on load and console log warnings.

Warning It seems the drive function no longer calls the class' observer could you have overridden that function?

Browser Testing

You can embed your tests and have them run in the background, if there is a plugin or ie version that doesn't do something this is a way to cloud source your testing.
You may find out ie 9 on windows server with a java plugin from 2002 can't load your site and now you have an idea of why.