.header_nav li {
list-style: none;
position: relative;
padding-bottom: 10px;
}
.header_nav li::before {
position: absolute;
content: '';
width: 72px;
height: 4px;
flex-shrink: 0;
background: #8B63FF;
bottom: 0;
left: 0;
border: 1px solid blue;
}