un esempio:

codice HTML:
...
<style type="text/css">
<!--
#mioDiv {
    background-color: #CC99FF; width:400px; height:300px;/* puramente indicativi */

    -webkit-animation: fadein 5s; 
    -moz-animation: fadein 5s;
    -ms-animation: fadein 5s;
    -o-animation: fadein 5s;
    animation: fadein 5s;
}

@keyframes fadein {
    from { opacity: 0; }
    to   { opacity: 1; }
}

@-moz-keyframes fadein {
    from { opacity: 0; }
    to   { opacity: 1; }
}

@-webkit-keyframes fadein {
    from { opacity: 0; }
    to   { opacity: 1; }
}

@-ms-keyframes fadein {
    from { opacity: 0; }
    to   { opacity: 1; }
}​

@-o-keyframes fadein {
    from { opacity: 0; }
    to   { opacity: 1; }
}​
-->
</style>
</head>

<body>
<div id="mioDiv"></div>
...