SOURCE

<div class="main">
    <div class="wave"><span>loading...</span></div>
</div>

body{
    display: flex;
    justify-content: center;
    align-items: center;
    width: 100vw;
    height: 100vh;
}
.main{
    width: 200px;
    height: 200px;
    border: 1px solid #3498db;
    border-radius: 50%;
    padding: 10px;
}
.wave{
    width: 200px;
    height: 200px;
    line-height: 200px;
    text-align: center;
    background: #3498db;
    border-radius: 50%;
    position: relative;
    border: transparent;
    overflow: hidden;
}
.wave span{
    color: white;
    text-transform: uppercase;
}
.wave::before{
    content: '';
    position: absolute;
    left: -50px;
    top: -150px;
    width: 300px;
    height: 300px;
    background: white;
    border-radius: 40%;
    animation: animate 2s linear infinite;
}
@keyframes animate{
    from{
        transform: rotate(0deg);
        /* top: -100px; */
    }
    to{
        transform: rotate(360deg);
        /* top: -300px; */
    }
}