/* Desktop first par défaut */
body {
    background-color: #f00;
}
#desktop, #tablette, #mobile {
    width: 80%;
    height: 500px;
    background-color: #ff0;
    font-size: 80px;
    padding: 30px;
    text-align: center;
}
#tablette, #mobile {
    display: none;
}

/*Mobile first*/
@media screen and (max-width: 800px) {
    /*Ecran de 500 à 800px */
    body {
        background-color: #00f;
    }  
    #tablette{
        display: block;
        background-color: #fff;
        font-size: 50px;
    }
    #desktop, #mobile {
        display: none;
    }
}


@media screen and (max-width: 500px) {
    /*Ecran inférieur à 500px */
    body {
        background-color: #000;
    } 
    #mobile{
        display: block;
        background-color: yellowgreen;
        font-size: 30px;
    }
    #desktop, #tablette {
        display: none;
    }
}