transition in Yahoo Mail

Before

.button {
  background-color: #6d28d9;
  transition: background-color 0.2s;
}
.button:hover {
  background-color: #5b21b6;
}

After

.button {
  /* Use the most visually appealing state as default.
     :hover is only supported in a few clients. */
  background-color: #6d28d9;
  color: #ffffff;
  text-decoration: none;
  font-weight: bold;
}