css how to check media query applies code example

Example 1: media query

@media only screen and (max-width: 600px) {
  body {
    background-color: lightblue;
  }
}

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); }
}