In your example, the buttons do not stay depressed. They stay focused. If you want to see the difference, do the following:
- Click and hold on a button.
- Release. You will see that when you release the mouse the button's appearance changes slightly, because it is no longer pressed.
If you do not want your buttons to stay focused after being released you can instruct the browser to take the focus out of them whenever you release the mouse.
Example
This example uses jQuery but you can achieve the same effect with vanilla JavaScript.
$(".btn").mouseup(function(){
$(this).blur();
})