/* code that is here, until the first @media block, will apply to any screen size */
#somethingorother {
    width: 800px;
}

@media screen and (max-width: 320px) {
    /* comes into effect for screens less than or equal to 320 pixels */
    #somethingorother {
        width: 120px;
    }
}

@media screen and (min-width: 321px) and (max-width: 480px) {
    /* comes into effect for screens between 321 and 480 pixels (inclusive) */
    #somethingorother {
        width: 320px;
    }
}

@media screen and (min-width: 481px) {
    /* comes into effect for screens larger than or equal to 481 pixels */
    #somethingorother {
        width: 480px;
    }
}

/* code that is here will apply to any screen size */