.source-link {
  z-index: 1;
  position: fixed;
  left: 0.5rem;
  top: 0.5rem;
  font-family: sans-serif;
  background-color: white;
  color: blue;
  padding: 0.25rem;
}
.source-link:hover {
  background-color: lightgray;
}
.source-link:active {
  background-color: gray;
}
.source-link:focus {
  outline: 1px solid blue;
}