@keyframes heartbeat {
  0% {
    transform: translate(-50%, -50%) scale(1);
  }
  50% {
    transform: translate(-50%, -50%) scale(1.5);
  }
  100% {
    transform: translate(-50%, -50%) scale(1);
  }
}

div.ball {
  transform: translate(-50%, -50%);

  display: inline-block;
  width: 50px;
  height: 50px;
  border-radius: 50%;
  border-style: solid;
  border-width: 1px;
  border-color: #333;
  background-color: rgba(0, 0, 0, 0);
  position: absolute;
  top: 0;
  left: 0;
  pointer-events: none;

/*   mix-blend-mode: difference;
 */
}
div.inner-ball {
  background-color: #333;
  width: 8px;
  height: 8px;
  border-radius: 50%;

  position: absolute;
  pointer-events: none;

  transform: translate(-50%, -50%);

/*   mix-blend-mode: difference;
 */
 z-index: 1;
}

/* h1:hover.inner-ball {
  animation: heartbeat 2s infinite;
}

a h1:hover ~ .inner-ball {
  transform: scale(5);
} */

.is-active {
  transform: scale(2);
  background: red;
}