html, body {
  overflow: hidden;
  margin: 0;
  padding: 0;
}

canvas {
  margin-top: auto;
  margin-bottom: auto;
  margin-left: auto;
  margin-right:auto;
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}
.bouton{
    position:absolute;
    top:15px;
    left:15px;
}

.bouton img{
    width:40px;;
}