```css
/* styleContact.css / ISELH */

/* =========================
   RÈGLES GÉNÉRALES
   ========================= */

* {
    box-sizing: border-box;
}

html {
    min-height: 100%;
}

body {
    margin: 0;
    padding: 0;
    font-family: Arial, sans-serif;
    color: #222;
    background-color: #f5f5f3;
}

main {
    width: 100%;
    padding: 30px 20px;
}

h1 {
    text-align: center;
    max-width: 800px;
    margin: 0 auto 30px auto;
    line-height: 1.2;
}


/* =========================
   FORMULAIRE
   ========================= */

#contact {
    width: 100%;
    max-width: 800px;
    margin-left: auto;
    margin-right: auto;
}

.form-group {
    display: grid;
    grid-template-columns: 150px minmax(0, 1fr);
    gap: 15px;
    align-items: center;
    margin-bottom: 20px;
}

.form-group-message {
    align-items: start;
}

.form-group label {
    font-weight: bold;
}

.form-group-message label {
    padding-top: 8px;
}


/* =========================
   CHAMPS
   ========================= */

.form-group input[type="email"],
.form-group input[type="text"],
.form-group textarea {
    width: 100%;
    box-sizing: border-box;
    padding: 8px 10px;
    border: 1px solid #bbb;
    border-radius: 4px;
    background-color: white;
    font-family: inherit;
    font-size: 1rem;
}

.form-group input[type="email"],
.form-group input[type="text"] {
    min-height: 40px;
}

.form-group textarea {
    min-height: 200px;
    resize: vertical;
    line-height: 1.5;
}


/* =========================
   BOUTON
   ========================= */

.form-actions {
    margin-top: 25px;
    margin-left: 165px;
}

#msg_envoyer {
    padding: 10px 20px;
    border: 1px solid #888;
    border-radius: 4px;
    background-color: #eee;
    color: #222;
    font-family: inherit;
    font-size: 1rem;
    cursor: pointer;
}

#msg_envoyer:hover {
    background-color: #ddd;
}


/* =========================
   BOUTON RETOUR
   ========================= */

.logo-retour {
    position: fixed;
    right: 20px;
    bottom: 20px;
    z-index: 10;
}

.logo-retour img {
    display: block;
    width: 70px;
    height: 70px;
    cursor: pointer;
}


/* =========================
   TABLETTE
   701px → 900px
   ========================= */

@media (min-width: 701px) and (max-width: 900px) {

    main {
        padding: 25px 20px;
    }

    h1 {
        max-width: 100%;
        margin-bottom: 25px;
    }

    #contact {
        max-width: 100%;
    }

    .form-group {
        grid-template-columns: 130px minmax(0, 1fr);
        gap: 12px;
    }

    .form-actions {
        margin-left: 142px;
    }
}


/* =========================
   MOBILE
   ≤ 700px
   ========================= */

@media (max-width: 700px) {

    main {
        padding: 20px 15px;
    }

    h1 {
        max-width: 100%;
        margin: 0 auto 25px auto;
        font-size: 1.6rem;
        line-height: 1.25;
    }

    #contact {
        max-width: 100%;
    }

    /*
       Sur mobile, chaque label passe
       au-dessus de son champ.
    */
    .form-group {
        display: flex;
        flex-direction: column;
        align-items: stretch;
        gap: 7px;
        margin-bottom: 20px;
    }

    .form-group label {
        padding: 0;
    }

    .form-group-message label {
        padding-top: 0;
    }

    .form-group input[type="email"],
    .form-group input[type="text"],
    .form-group textarea {
        width: 100%;
        max-width: 100%;
        font-size: 1rem;
    }

    .form-group textarea {
        min-height: 200px;
    }

    /*
       Le bouton prend toute la largeur
       disponible sur mobile.
    */
    .form-actions {
        margin: 25px 0 0 0;
    }

    #msg_envoyer {
        display: block;
        width: 100%;
        min-height: 48px;
        padding: 10px 15px;
        font-size: 1rem;
    }

    /*
       Le bouton retour reste fixe,
       mais légèrement plus petit.
    */
    .logo-retour {
        right: 12px;
        bottom: 12px;
    }

    .logo-retour img {
        width: 60px;
        height: 60px;
    }
}


/* =========================
   PETITS MOBILES
   ≤ 400px
   ========================= */

@media (max-width: 400px) {

    main {
        padding: 15px 10px;
    }

    h1 {
        font-size: 1.4rem;
        margin-bottom: 20px;
    }

    .form-group {
        margin-bottom: 18px;
    }

    .form-group textarea {
        min-height: 180px;
    }

    #msg_envoyer {
        min-height: 50px;
    }

    .logo-retour {
        right: 8px;
        bottom: 8px;
    }

    .logo-retour img {
        width: 55px;
        height: 55px;
    }
}