CSS3 Spin Animation

Question

To use CSS3 Animation you must also define the actual animation keyframes (which you named spin)

Read https://developer.mozilla.org/en-US/docs/CSS/Tutorials/Using_CSS_animations for more info

Once you've configured the animation's timing, you need to define the appearance of the animation. This is done by establishing two or more keyframes using the @keyframes at-rule. Each keyframe describes how the animated element should render at a given time during the animation sequence.

---

Demo :

div {
    margin: 20px;
    width: 100px;
    height: 100px;
    background: #f00;
    -webkit-animation-name: spin;
    -webkit-animation-duration: 4000ms;
    -webkit-animation-iteration-count: infinite;
    -webkit-animation-timing-function: linear;
    -moz-animation-name: spin;
    -moz-animation-duration: 4000ms;
    -moz-animation-iteration-count: infinite;
    -moz-animation-timing-function: linear;
    -ms-animation-name: spin;
    -ms-animation-duration: 4000ms;
    -ms-animation-iteration-count: infinite;
    -ms-animation-timing-function: linear;
    
    animation-name: spin;
    animation-duration: 4000ms;
    animation-iteration-count: infinite;
    animation-timing-function: linear;
}
@-ms-keyframes spin {
    from { -ms-transform: rotate(0deg); }
    to { -ms-transform: rotate(360deg); }
}
@-moz-keyframes spin {
    from { -moz-transform: rotate(0deg); }
    to { -moz-transform: rotate(360deg); }
}
@-webkit-keyframes spin {
    from { -webkit-transform: rotate(0deg); }
    to { -webkit-transform: rotate(360deg); }
}
@keyframes spin {
    from {
        transform:rotate(0deg);
    }
    to {
        transform:rotate(360deg);
    }
}

<div></div>

@-moz-keyframes spin {
    from { -moz-transform: rotate(0deg); }
    to { -moz-transform: rotate(360deg); }
}
@-webkit-keyframes spin {
    from { -webkit-transform: rotate(0deg); }
    to { -webkit-transform: rotate(360deg); }
}
@keyframes spin {
    from {transform:rotate(0deg);}
    to {transform:rotate(360deg);}
}

You haven't specified any keyframes. I made it work here.

div {
    margin: 20px;
    width: 100px; 
    height: 100px;    
    background: #f00;
    -webkit-animation: spin 4s infinite linear;
}

@-webkit-keyframes spin {
    0%  {-webkit-transform: rotate(0deg);}
    100% {-webkit-transform: rotate(360deg);}   
}

You can actually do lots of really cool stuff with this. Here is one I made earlier.

:)

N.B. You can skip having to write out all the prefixes if you use -prefix-free.

As of latest Chrome/FF and on IE11 there's no need for -ms/-moz/-webkit prefix. Here's a shorter code (based on previous answers):

div {
    margin: 20px;
    width: 100px;
    height: 100px;
    background: #f00;

    /* The animation part: */
    animation-name: spin;
    animation-duration: 4000ms;
    animation-iteration-count: infinite;
    animation-timing-function: linear;
}
@keyframes spin {
    from {transform:rotate(0deg);}
    to {transform:rotate(360deg);}
}

Live Demo: http://jsfiddle.net/9Ryvs/3057/

HTML with font-awesome glyphicon.

<span class="fa fa-spinner spin"></span>

CSS

@-moz-keyframes spin {
    to { -moz-transform: rotate(360deg); }
}
@-webkit-keyframes spin {
    to { -webkit-transform: rotate(360deg); }
}
@keyframes spin {
    to {transform:rotate(360deg);}
}

.spin {
    animation: spin 1000ms linear infinite;
}

The only answer which gives the correct 359deg:

@keyframes spin {
  from { transform: rotate(0deg); }
  to { transform: rotate(359deg); }
}

&.active {
  animation: spin 1s linear infinite;
}

Here's a useful gradient so you can prove it is spinning (if its a circle):

background: linear-gradient(to bottom, #000000 0%,#ffffff 100%);