Demo:
A 'typical' Modal Window made with JavaScript & CSS

This modal window is created to demonstrate how poor keyboard accessibility is for many modal windows.

Try it out yourself. Use a mouse at first to see that the modal works just fine if you can point and click. Then try just interact with the modal by only using your keyboard.

You can see the source code for this demo on Codepen.

Here's a link to the wikipedia entry on modal windows.