form {
    font: 1em sans-serif;
    max-width: 320px;
}

p>label {
    display: block;
}

input[type="text"],
input[type="email"],
input[type="number"],
textarea,
fieldset {
    width: 100%;
    border: 1px solid #333;
    box-sizing: border-box;
}

input:invalid {
    box-shadow: 0 0 5px 1px red;
}

.invalid {
    box-shadow: 0 0 5px 1px red;
}

input:focus:invalid {
    box-shadow: none;
}