.progress {
    width: 100%;
    height: 4px;
    background: #ddd;
    overflow: hidden;
    position: relative;
  }
  
.progress > *{ 
    width: 25%;
    height: 100%;
    background: linear-gradient(90deg, #2196f3, #64b5f6);
    position: absolute;
    animation: progress 1s ease-in-out infinite;
    left: 0px;
}

@keyframes progress {
    0% {
        left: -50%;
        width: 40%;
    }  
 
    100% {
        left: 100%;
        width: 50%;
    } 
}