html, body {
  margin: 0;
  height: 100%;
  background: #fafaf7;
  overflow: hidden;
  touch-action: manipulation;
  user-select: none;
  -webkit-user-select: none;
  -webkit-tap-highlight-color: transparent;
  cursor: pointer;
}
#strobe {
  position: fixed;
  inset: 0;
  width: 100%;
  height: 100%;
}
#check {
  position: fixed;
  top: 50%;
  left: 50%;
  width: min(70vw, 70vh);
  height: min(70vw, 70vh);
  transform: translate(-50%, -50%);
  opacity: 0;
  pointer-events: none;
  fill: none;
  stroke: #22a34a;
  stroke-width: 7;
  stroke-linecap: round;
  stroke-linejoin: round;
}
#toggle {
  position: fixed;
  top: 50%;
  left: 50%;
  width: min(18vw, 18vh);
  height: min(18vw, 18vh);
  transform: translate(-50%, -50%);
  pointer-events: none;
  fill: #b5b5b0;
  transition: opacity 150ms;
}
#toggle .pause { display: none; }
body.running #toggle .play { display: none; }
body.running #toggle .pause { display: inline; }
body.checking #toggle { opacity: 0; }