form::after{
display: block;
content: '';
height: 5px;
position: absolute;
bottom: -5px;
left: 0;
background-color: red;
animation: line 3s linear infinite;
}
@keyframes line {
from {
width: 0%;
}
50%{
width: 100%;
}
to {
width: 0%;
}
}