anim.css

html,
body {
  height: 100%;
}

body {
  background-color: #4a4a4a;
  background: radial-gradient(#a88, #234);
  overflow: hidden;
}

:root {
  --margin: 0;
}

.container {
  position: absolute;
  top: var(--margin);
  bottom: var(--margin);
  left: var(--margin);
  right: var(--margin);
}

.nextStepButton {
  display: none;

  position: absolute;
  top: 0;
  left: 0;

  -webkit-appearance: none;
  user-select: none;

  font-size: 16px;
  outline: none;
  padding: .25em .5em;
  border-radius: 2px;
  cursor: pointer;

  border: 1px solid #000;
  box-shadow: 0 0 0 1px #fff;
  background-color: rgba(0, 0, 0, .5);
  color: #fff;
}

.nextStepButton:hover {
  box-shadow: 0 0 0 1px #000;
  background-color: #f0f0f0;
  color: #000;
}

.underline {
  text-decoration: underline;
}

canvas {
  width: 100%;
  height: 100%;
  object-fit: contain;
  image-rendering: pixelated;
}