Exemple d'une animation qui se déclenche au bout de 2 sec et qui ne se répète pas :

.animation {
    -webkit-animation: anim2s ease-in-out both;
    -moz-animation: anim 2s ease-in-out both;
    -o-animation: anim 2s ease-in-out both;
    -ms-animation: anim 2s ease-in-out both;
    animation: anim 2s ease-in-out both;
}

@-webkit-keyframes anim { 
    from {height: 50%; top: 50%;}
    to {height: 0; top: 0;}
}
@-moz-keyframes anim {
    from {height: 50%; top: 50%;}
    to {height: 0; top: 0;}
}
@-o-keyframes anim { 
    from {height: 50%; top: 50%;}
    to {height: 0; top: 0;}
}
@-ms-keyframes anim { 
    from {height: 50%; top: 50%;}
    to {height: 0; top: 0;}
}
@keyframes anim { 
    from {height: 50%; top: 50%;}
    to {height: 0; top: 0;}
}