.demo_box {
    border: 1px solid #3DA5DC;
    background: #a4dcf9;
    height: 100px;
    width: 200px;
    text-align: center;
    color: #fff;
}
.demo_box{
    -webkit-animation:f1 2s 0.5s 1  linear;
    -moz-animation:f1 2s 0.5s 1  linear;
    position:relative;
    left:10px;
    width:100px;
    height:100px;
    margin:10px 0;
    overflow:hidden;
}
.forwards{ 
    -webkit-animation-fill-mode:forwards;
    -moz-animation-fill-mode:forwards;
}
.backwards{
    -webkit-animation-fill-mode:backwards;
    -moz-animation-fill-mode:backwards;
}
@-webkit-keyframes f1{
    0%{left:10px;}
    100%{left:500px;}
}
@-moz-keyframes f1{
    0%{left:10px;}
    100%{left:500px;}
}