<style>
a {
    position: relative;
    --text-opacity: 1;
    color: #ed64a6;
    color: rgba(237,100,166,var(--text-opacity));
    display: inline-block;
    text-decoration: none;
    transition: all .2s ease
}

a:before {
    position: absolute;
    bottom: 0;
    left: 0;
    right: 0;
    --bg-opacity: 1;
    background-color: #000;
    background-color: rgba(0,0,0,var(--bg-opacity));
    content: "";
    transform-origin: bottom right;
    transform: scaleX(0);
    height: .125rem;
    transition: transform .5s ease
}

a:hover:before {
    transform-origin: bottom left;
    transform: scaleX(1)
}

a:hover {
    color: var(--color-text)
}
</style>
<a href="#">将鼠标移动到我这里，然后移开看看效果</a>