html {
    background-color: bisque;
}

body {
    width: 1200px; /* base=100% para el form */
}

h1 {
    text-align: center;
}

form {
    background-color: blanchedalmond;
    /* Base: 1200px = 100% */
    width: 30.666667%;        /* 368×100/1200 */
    margin: 1.666667% auto;   /* 20×100/1200 */
    padding: 1.25%;           /* 15×100/1200 */
    border-radius: 3.75%;     /* 15×100/400 (ancho total del form) */
    border: 1px solid black;
}

label {
    font-family: 'Gill Sans', 'Gill Sans MT', Calibri, 'Trebuchet MS', sans-serif;
    color: black;
    display: inline-block;
    /* Base: 368px = 100% (contenido del form) */
    margin-bottom: 2.717391%; /* 10×100/368 */
}

input[type="text"],
textarea {
    width: 54.347826%;        /* 200×100/368 */
    padding: 2.173913%;       /* 8×100/368 */
    border: 1px solid #888;
    margin-bottom: 4.076087%; /* 15×100/368 */
    font-family: inherit;
}

textarea {
    height: 80px; /* height en % no funciona con un padre en auto */
}

button,
input[type="submit"] {
    width: 26.630435%;        /* 98×100/368 */
    padding: 2.717391%;       /* 10×100/368 */
    margin-top: 2.717391%;    /* 10×100/368 */
    background-color: #d2b48c;
    border: 1px solid black;
    border-radius: 4.166667%; /* 5×100/120 (ancho total del botón) */
}

input[type="checkbox"] {
    width: 4.347826%;         /* 16×100/368*/
    margin-bottom: 2.717391%; /* 10×100/368 */
    margin-left: 2.717391%;   /* 10×100/368 */
    height: 16px;
    display: inline-block;
}