<!DOCTYPE html>
<style>
canvas {
    width: 200px;
    height: 200px;
}
</style>
<canvas></canvas>
<script>
function paint() {
    const canvas = document.querySelector("canvas");
    canvas.width = 200;
    canvas.height = 200;
    const ctx = canvas.getContext("2d");
    ctx.fillStyle = "rgb(0, 255, 0)";
    ctx.fillRect(0, 0, 200, 200);
}
window.addEventListener("load", paint, false);
</script>