form {
    width: 600px;
    padding: 20px;
    border: 3px solid pink;
    margin: 30px auto;
    display: flex;
    flex-direction: column;
    padding-top: 50px;
    position: relative;       /* 相对定位 */
}

form > span {
    position: absolute;
    left: 50%;
    transform: translateX(-50%);
    top: 5px;
    font-size: 20px;
    color: red;
    display: none;
}

form > label {
    height: 50px;
    font-size: 30px;
}

form > label > input {
    width: 300px;
    padding-left: 20px;
    font-size: 30px;
    height: 40px;
}

form > button {
    padding: 5px;
    font-size: 30px;
    cursor: pointer;           /* 光标为一只手 */
}