<canvas id="myCanvas">
Your browser does not support the canvas tag.
</canvas>

<script>
let c = document.getElementById("myCanvas");
let ctx = c.getContext("2d");
ctx.fillStyle = "red";
ctx.fillRect(20, 20, 75, 50);
//Turn transparency on
ctx.globalAlpha = 0.2;
ctx.fillStyle = "blue";
ctx.fillRect(50, 50, 75, 50);
ctx.fillStyle = "green";
ctx.fillRect(80, 80, 75, 50);
</script>