<div class="half">
  <canvas width="100" height="100"></canvas>
  <div>
    left
  </div>
</div>
<div>
  right
</div>

html, body {
  width: 100%;
  height: 100%;
  margin: 0;
  padding: 0;
}
.half {
  float: left;
  background: gray;
  height: 50%;
  position: relative;
}
.half > canvas {
  height: 100%;
  z-index: -1;
  opacity: 0;
}
.half > div {
  position: absolute;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
}