table {
  border: 1px solid #AAAAAA;
  border-collapse: collapse;
  padding: 5px;
}
table th {
  border: 1px solid #AAAAAA;
}
table td {
  border: 1px solid #AAAAAA;
  padding: 5px;
}
.left {
  text-align: left;
}
.center {
  text-align: center;
}
<style>
/* Tooltip container */
.tooltip {
  position: relative;
  display: inline-block;
}

/* Tooltip text */
.tooltip .tooltiptext {
  visibility: hidden;
  width: 120px;
  background-color: lightgrey;
  color: #000;
  text-align: center;
  padding: 5px 0;
  border-radius: 6px;
 
  /* Position the tooltip text - see examples below! */
  position: absolute;
  z-index: 1;
}

/* Show the tooltip text when you mouse over the tooltip container */
.tooltip:hover .tooltiptext {
  visibility: visible;
}
</style>