Deque Systems makes an open source project called aXe:

The Accessibility Engine for automated testing of HTML-based user interfaces. Drop the aXe on your accessibility defects!

There are a variety of ways you can use it, like a browser extension for Chrome or Firefox. It's also just a stand-alone library, which is a smart move as it means the functionality of aXe is portable to all these different places.

To use it on a Pen, it's as simple as including the library, like as an External Resource:

Then calling it in your JavaScript:, results) {

Here's an example Pen where you can see it find errors.