css media page code example

Example 1: css media screen

@media (max-width: 991px){
    .mobile{
        display: block;
    }
}

Example 2: media query in html style

span { background-image: url(particular_ad.png); }

@media (max-width: 300px) {
    span { background-image: url(particular_ad_small.png); }
}