form {
    border: 3px solid black;
    padding: 10px;
    margin-left: auto;
    margin-right: auto;
    margin-top: auto;
    margin-bottom: auto;
    display: flex;
    justify-content: center;
    flex-direction: column;
    width: 20%;
}

input:valid {
    background-color: lightgreen;
}

input:invalid {
    background-color: lightcoral;
}