<meta name="viewport" content="width=device-width, initial-scale=1.0">

body {
    /* big screens */
    background-color: red;
}

@media screen and (max-width: 900px) {
    body {
      /* medium screens */
      background-color: yellow;
    }
}

@media screen and (max-width: 400px) {
    body {
        /* small screens */
        backgund-color: green;
    }
}

body {
    font-size: 2vw;
}

img {
    width: 100%;
}