简单版
border: 0; outline: 0; background: transparent; border-bottom: 2px solid black; width: 20px;
稍微复杂点，漂亮点的
background-color: transparent; border: none; border-bottom: 2px solid #fff; outline: none; padding-bottom: 10px; text-align: center; width: 400px; font-size: 300%; color: #fff; font-weight: 100;