DIV#illusion {
   font-size: 1px;
   background: #000;
   width: 214px;
   padding: 0;
   border: 24px solid #000;
   margin: 30px auto;
   }

DIV#illusion DIV {
   background: #ccc;
   padding: 0;
   border-color: #fff #000;
   border-style: solid;
   margin: 0 auto;
   }

DIV#illusion:hover {
   font-size: 1px;
   background: #fff;
   width: 214px;
   padding: 0;
   border: 24px solid #fff;
   margin: 30px auto;
   }

DIV#illusion:hover DIV {
   background: #000;
   padding: 0;
   border-color: #000 #fff;
   border-style: solid;
   margin: 0 auto;
   }