Tip #13: Provide visible indication of focus

a { 
  color: black;
  background-color: white;
}
a:hover {   
  color: white;
  background-color: black;
}

Focus is especially important for keyboard users

a { 
  color: black;
  background-color: white;
}
a:hover, a:focus {   
  color: white;
  background-color: black;
}