Suppose you wrote: @@media only screen and (min-width : 480px) {}. You should have had only one @ symbol before a media query. Yet this is the kind of error that is easy to overlook. A misplaced curley brace or missing semicolon can also cause serious problems, yet be easy to overlook.

The cure is to run your CSS through a validator. It would have found this error for you.

  • Validators typically support at least two modes:
    • One involves checking a URL from a site on the Internet.
    • If your site is on your home machine, or on an Intranet, then you can’t pass in a URL. Instead you have to paste in your code.