title: Apply CSS transition when element comes into viewport
body: <p>I'm trying to apply CSS transition effect when an element comes to viewport (i.e. when user scrolls to it), but not before.</p>
  
  <p>I already know how to use CSS transitions, but how do I apply them only when the element comes to viewport?</p>
  
  <p>What it the best way of doing this? If there's some library's to simplify the task, I would be glad to know.</p>