h2{
    font-weight: normal;
    font: size 35px;
    position:relative;
    margin: 40px 0;
}
h2::before{
    content:'';
    position:absolute;
    width:100px;
    height: 3px;
    background-color: green;
    bottom: -10px;
    left:50%;
    transform:translate(-50%);
    animation: animate 4s linear infinite;
}
@keyframes animate {
    0%{
        width: 100px;
    }
    50%{
        width: 200px;
    }
    100%{
        width: 100px;
    }
    
}