a[data-tooltip]:link, a[data-tooltip]:visited {
  position: relative;
  text-decoration: none;
  border-bottom: solid 1px;
}

a[data-tooltip]:before {
  content: "";
  position: absolute;
  border-top: 1em solid #0090ff;
  border-left: 1.5em solid transparent;
  border-right: 1.5em solid transparent;
  display: none;
  top: -1em;
}

a[data-tooltip]:after {
  content: attr(data-tooltip);
  position: absolute;
  color: white;
  top: -2.5em;
  left: -1em;
  background: #0090ff;
  padding: .25em .75em;
  border-radius: .5em;
  white-space: nowrap;
  display: none;
}

a[data-tooltip]:hover:before, a[data-tooltip]:hover:after {
  display: inline;
}

Code language: CSS (css)