<ul>
    <li>不可思议的CSS</li>
    <li>导航栏</li>
    <li>光标下划线跟随</li>
    <li>PURE CSS</li>
    <li>Coco</li>
</ul>

ul {
    display: flex;
    position: absolute;
    width: 800px;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

li {
    position: relative;
    padding: 20px;
    font-size: 24px;
    color: #000;
    line-height: 1;
    transition: 0.2s all linear;
    cursor: pointer;
}

li::before {
    content: "";
    position: absolute;
    top: 0;
    left: 100%;
    width: 0;
    height: 100%;
    border-bottom: 2px solid #000;
    transition: 0.2s all linear;
}

li:hover::before {
    width: 100%;
    top: 0;
    left: 0;
    transition-delay: 0.1s;
    border-bottom-color: #000;
    z-index: -1;
}

li:hover ~ li::before {
    left: 0;
}

li:active {
    background: #000;
    color: #fff;
}