<h1 id="logo" class="run-animation">
  Fancy Text (click to restart)
</h1>

@keyframes my-animation {        
  from {
    opacity : 0;
    left : -500px;
  }
  to {
    opacity : 1;
    left : 0;
  }      
}

.run-animation {
  position: relative;
  animation: my-animation 2s ease;
}

#logo {
  text-align: center;
  font-family : Palatino Linotype, Book Antiqua, Palatino, serif;
}

// This changes everything
"use strict";

// retrieve the element
var element = document.getElementById("logo");

// reset the transition by...
element.addEventListener("click", function(e){
  e.preventDefault;
  
  // -> removing the class
  element.classList.remove("run-animation");
  
  // -> triggering reflow /* The actual magic */
  // without this it wouldn't work. Try uncommenting the line and the transition won't be retriggered.
  // This was, from the original tutorial, will no work in strict mode. Thanks Felis Phasma! The next uncommented line is the fix.
  // element.offsetWidth = element.offsetWidth;
  
  void element.offsetWidth;
  
  // -> and re-adding the class
  element.classList.add("run-animation");
}, false);