<input type="checkbox" id="slowmo" />
<label for="slowmo">Slowmo!</label>
<div>🐶</div>

body { font-size: 32px; }
input { height: 32px; width: 32px; }
div { font-size: 64px; width: fit-content; }

@property --angle1 {
  syntax: "<angle>";
  initial-value: 0deg;
  inherits: false;
}
@property --angle2 {
  syntax: "<angle>";
  initial-value: 0deg;
  inherits: false;
}

div {
  position: absolute;
  animation-name: spin1;
  animation-delay: 0s;
  animation-duration: 2s;
  animation-timing-function: linear;
  animation-iteration-count: infinite;
  transform-origin: 100% 100%;
  transform: rotate(var(--angle1));
}

#slowmo:checked ~ div {
  transform: rotate(var(--angle2));
  animation-name: spin1, spin2;
  animation-duration: 2s, 5s;
  animation-play-state: paused, running;
}

@keyframes spin1 {
  to {
    --angle1: 360deg;
  }
}

@keyframes spin2 {
  from {
    --angle2: var(--angle1);
  }
  to {
    --angle2: calc(var(--angle1) + 360deg);
  }
}