My CSS says that my width is 80%, my default background is #456 and my links are white.
My JSS says that if my width exceeds 550px, I turn red and my links turn yellow.
If I measure less than 550px, I return to normal.
Resize browser to change my size and see the magic happen!
JSS code:

<script type="text/jss">

@element "#widget" and (min-width: 550px){
  #widget {
    background: red;
  }
  #widget a {
    background: yellow;
  }
}

</script>