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HTML

            
              <h1>Synching Animation Clocks with the Web Animation API</h1>
<p>This demo was made for <a href="http://www.smashingmagazine.com/?p=210830">my post on Smashing Magazine about where web animation is headed.</a> It uses a <a href="https://github.com/web-animations/web-animations-js">polyfill</a> to enable the Web Animation API. (Psst: Check out the JavaScript and CSS to see how it's done.) To learn more about the Web Animation API, <a href="http://rachelnabors.com/waapi/">check out my WAAPI resource page!</a></p>
<div id="CSSCats">
  <h2>Cats animated with <br />CSS Animations</h2>
  <p>Currently, it is impossible to synch two separate animations with CSS animations. Try adding new cats animated with CSS below. Notice that they do not run in synch with each other.</p>
  <div class="cat withCSS"></div>
  <button onclick="animateNewCatWithCSS()">Add a Cat</button>
</div>

<div id="WAAPICats">
  <h2>Cats animated with the <br />Web Animation API</h2>
  <p>But with the Web Animation API, you're able to set the <code>startTime</code> value across animations to properly synch them. When you add cats with the Web Animation API, they run lockstep!</p>
  <div class="cat" id="withWAAPI"></div>
  <button onclick="animateNewCatWithWAAPI()">Add a Cat</button>
</div>
!

CSS

            
              /* All cats have the same dimensions and the same sprite for a background image. */
.cat {
  background: url(http://stash.rachelnabors.com/24ways2012/cat_sprite.png) -800px 0 no-repeat; 
  height: 200px; width: 400px;
}

/* The cats animated with CSS have their running animations set with CSS */
.cat.withCSS {
  animation: run-cycle;
  animation-duration: .75s;
  animation-timing-function: steps(13,end);
  animation-iteration-count: infinite;
}

/* The keyframes for the run cycle.
   This moves the background image sprite around. */
@keyframes run-cycle {  
  0% {background-position: -800px 0; }
  100% {background-position: -800px -2591px; } 
}

/* More information on this spriting technique in my 24 Ways article: 
http://24ways.org/2012/flashless-animation/ 
*/

/* It's all presentational from here on. 
   Nothing to see here! */
#CSSCats, #WAAPICats {
  display: inline-block;
  text-align: center;
  vertical-align: top;
  width: 400px;
}

@media only screen and (min-width: 850px) {

  #CSSCats, #WAAPICats {
    margin: 3em 1em;
  }
}

#CSSCats p, #WAAPICats p {
  text-align: left;  
}

body { 
  background: #e5e6e9;
  color: #071933;
  font-family: 'Lato', sans-serif;
  font-size: 1em; 
  line-height: 1.5em;
  margin: 1em 1em;
  text-align: center;
  width: 100%;
}

@media only screen and (min-width: 850px) { 
  body {
    margin: 4em auto;
    width: 80%;
  }
}
h1 {
  font-family: 'Aleo', sans-serif;
  font-weight: 200;
  font-size: 2em;
  line-height: 1.25em;
}

h2 {
  font-family: 'Aleo', sans-serif;
  font-weight: 400;
  font-size: 1.5em;
  line-height: 1.25em;  
}

p {
  margin: .5em 0;
}

button {
  background: #2d0d3c;
  border: 0;
  color: #f7e6ff;
  cursor: pointer;
  font-size: .9em;
  padding: .7em 2em;
}

@media only screen and (min-width: 850px) { 
  button {
    padding: 1em 3em;
    font-size: 1em;
  }
}

code { font-family: monospace;}

a:link { color: #1954c0; } a:visited {color: #702096; }
a:hover, a:active, a:focus {
  color: #646d97;
}
!

JS

            
              // Here we set up a keyframes object containing values set in an '@keyframes' CSS rule
var keyframes = [
  {backgroundPosition: "-800px 0"},
  {backgroundPosition: "-800px -2591px"}
];
// and a timing object, which contains values we'd normally put in the 'animation' CSS rule
var timing = {
  duration: 750,
  iterations: Infinity,
  easing: "steps(13, end)"
};

/* with the WAAPI's animate function, we apply both keyframes and timing objects to .cat#withWAAPI 
(Have a look in your DOM inspector!)*/
var catRunning = document.getElementById ("withWAAPI").animate(keyframes, timing);

/* A function that makes new cats! 
Goodie! This will be useful below. */
function addCat(){
  var newCat = document.createElement("div");
  newCat.classList.add("cat");
  return newCat;
}

/* This is the function that adds a cat to the CSS column */
function animateNewCatWithCSS() {
  // make a new cat
  var newCat = addCat();
  // Add the CSS class that kicks off the CSS animation
  newCat.classList.add("withCSS");
  CSSCats.appendChild(newCat);
}

/* This is the function that adds a cat to the WAAPI column */
function animateNewCatWithWAAPI() {
  // make a new cat
  var newCat = addCat();
  // animate said cat with the WAAPI's "animate" function
  var newAnimationPlayer = newCat.animate(keyframes, timing);
  // set that player object's start time to be the same as the original .cat#withWAAPI
  newAnimationPlayer.startTime = catRunning.startTime;
  WAAPICats.appendChild(newCat);
}
!
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