<!DOCTYPE html>
<html>
<head></head>
<body>
<canvas id="canvas_id" width="300" height="300"></canvas>
<script>
var canvas = document.getElementById("canvas_id");
var ctx = canvas.getContext("2d");
ctx.fillStyle = "rgb(0, 255, 0)"; // green
ctx.fillRect(50, 50, 200, 200);
/*
run:
draw and fill rectangle with green color
*/
</script>
</body>
</html>