#content {
  position: absolute;
  width: auto;
  height: auto;
  left: 50%;
  top: 50%;
  z-index: 1;
  transform: translate(-50%, -50%);
}

.centerbox {
  /* flexbox, por favor */
  display: -webkit-box;
  -webkit-box-orient: horizontal;
  -webkit-box-pack: center;
  -webkit-box-align: center;
  display: -moz-box;
  -moz-box-orient: horizontal;
  -moz-box-pack: center;
  -moz-box-align: center;
  width: 100%;
  height: 100%;
  margin: 0; padding: 0;
}
body, html { width: 100%; height: 100%; padding: 0; margin: 0; }
canvas {
  /* border-radius: 30px;  Border radiuses don't seem to work with putImageData */
  box-shadow: 0 0 10px #777;
  width: 1024px;
  height: 768px;
}
body {
  background-color: #eee;
}