/* Hover over tooltip */
.tooltip {
  position:absolute;        /* Ensure that it does not take space on the web page */
  visibility:hidden;        /* Normally it is invisible */
  background-color:yellow;  /* Something that contrasts with the rest of the web page */
  border-color:#000;        /* Black border, 1px wide */
  border-width:1px;
  border-style:solid;
  z-index:12;               /* Ensure that it hides anything that it where it pops up */
}
a:hover + .tooltip {   /* Select for the item (span) that follow a link (<a>) */
                            /* that the mouse is hovering over */
  visibility:visible;       /* Make the text in class tooltip_demo visible */
}