<!DOCTYPE html>
<html>
<style>
html, body { margin: 0; padding: 0; }
</style>
<body>
<canvas id ="canvas" width="50" height="20" style="position:relative; top:40px; left:50px"></canvas>
<script>
var canvas = document.getElementById('canvas');
var context = canvas.getContext("2d");
context.fillStyle = 'green';
context.fillRect(0, 0, 50, 20);
</script>
</body>
</html>