.loader{
    display: block;
    position: relative;
    height: 12px;
    width: 100%;
    border: 1px solid #fff;
    border-radius: 10px;
    overflow: hidden;
}
.loader:after{
    content: '';
    position: absolute;
    left: 0;
    top: 0;
    height: 100%;
    width: 0;
    background: #5e5e5e;
    animation: 6s prog ease-in infinite;
}

@keyframes prog {
    to  {   width: 100%;}
}