Keyframes

<div class="main">
    <div class="box"></div>
</div>

.main {
    width: 100px;
    border: 1px solid #ffffff;
    height: 5px;
    border-radius: 10px;
    z-index: 2px;
    padding: 2px;
}
.box {
    border-radius: 10px;
    width: 100px;
    height: 5px;
    background-color: #13aaaa;
    animation: anima;
    animation-duration: 4s;
    animation-iteration-count: infinite;
}
@keyframes anima {
    from {
        width: 0px;
    }
    to {
        width: 100px;
    }
}