<!DOCTYPE html>
<html>
<body>
<canvas id ="canvas" width="500" height="500"></canvas>
<script>
var canvas = document.getElementById('canvas');
var context = canvas.getContext("2d");
context.fillStyle = 'rgba(75, 125, 0, 0.8)';
context.fillRect(0, 0, 500, 500);
</script>
</body>
</html>