Untitled

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        document.addEventListener('DOMContentLoaded', function(){
            const canvas = document.getElementById("canvasId");
            const context = canvas.getContext("2d");

            context.fillStyle = 'green';

        });
    </script>
    <style>
        #canvasId{
            background:black;
        }
    </style>
</head>
<body>
    <canvas id="canvasId" width="800" height="600"></canvas>
</body>
</html>