You can run a CSS animation on page load without using any JavaScript; you just have to use CSS3 Keyframes.

Let’s Look at an Example…

Here’s a demonstration of a navigation menu sliding into place using CSS3 only:

@keyframes slideInFromLeft {
  0% {
    transform: translateX(-100%);
  }
  100% {
    transform: translateX(0);
  }
}

header {  
  /* This section calls the slideInFromLeft animation we defined above */
  animation: 1s ease-out 0s 1 slideInFromLeft;
  
  background: #333;
  padding: 30px;
}

/* Added for aesthetics */ body {margin: 0;font-family: "Segoe UI", Arial, Helvetica, Sans Serif;} a {text-decoration: none; display: inline-block; margin-right: 10px; color:#fff;}

<header>
  <a href="#">Home</a>
  <a href="#">About</a>
  <a href="#">Products</a>
  <a href="#">Contact</a>
</header>

Break it down…

The important parts here are the keyframe animation which we call slideInFromLeft…

@keyframes slideInFromLeft {
    0% {
        transform: translateX(-100%);
    }
    100% {
        transform: translateX(0);
    }
}

…which basically says “at the start, the header will be off the left hand edge of the screen by its full width and at the end will be in place”.

The second part is calling that slideInFromLeft animation:

animation: 1s ease-out 0s 1 slideInFromLeft;

Above is the shorthand version but here is the verbose version for clarity:

animation-duration: 1s; /* the duration of the animation */
animation-timing-function: ease-out; /* how the animation will behave */
animation-delay: 0s; /* how long to delay the animation from starting */
animation-iteration-count: 1; /* how many times the animation will play */
animation-name: slideInFromLeft; /* the name of the animation we defined above */

You can do all sorts of interesting things, like sliding in content, or drawing attention to areas.

Here’s what W3C has to say.