I’ve found it handy during the development of responsive CSS to know what stylesheet or media query is active.

@media screen and (max-width:600px) {
  .site-time p::after { 
    content:" (0-600px)";
  }
}

@media (min-width: 601px) and (max-width: 800px) {
  .site-time p::after { 
    content:" (601-800px)";
  }
}

I’ve picked an element that was in every variant of the site layout, and appended some text indicating the active sheet.