body {
    font-family: Arial, Helvetica, sans-serif;
}

section {
    width: 60%;
    margin-left: 20%
}

fieldset {
    width: 90%;
}

.form-group {
    width: 100%;
}

@media screen and (max-width: 800px) {
    section {
        width: 100%;
        /* The width is 100%, when the viewport is 800px or smaller */
        margin: auto;
    }
}