html, body {
  min-height: 100%;
  padding: 0;
  margin: 0;
}

canvas{
  position: absolute;
  top: 0;
  left: 0;
  padding: 0;
  margin: 0;
  background: #fff;
}

#parent {position: relative; width: 100%; height: 100vh;}

#child {
    position: absolute;
    top: 50%;
    left: 50%;
    height: 240px;
    width: 240px;
    margin: -160px 0 0 -120px;
}