@media only screen and (max-width: 500px) { 
  table, tr, td {
    display: block; /* table-cell -> block */
    width: 100%; 
  }	
  
  .main-col { 
    display: table-header-group; 
  }
  
  .sub-col { 
    display: table-footer-group; 
  }					
}