Javascript でファビコン画像を生成してます

<script>
// canvas
const canvas = document.createElement("canvas");
canvas.width = 32;
canvas.height = 32;
const ctx = canvas.getContext("2d");

// circle
ctx.beginPath();
ctx.arc(canvas.width / 2, canvas.width / 2, canvas.width / 2, 0, Math.PI * 2, true);
ctx.fillStyle = '#333';
ctx.fill();

// text
const faiconText = new Date().getDate();
ctx.fillStyle = 'white';
ctx.font = '22px sunserif bold';
ctx.textAlign = 'center';
ctx.textBaseline = 'middle';
ctx.fillText(faiconText, canvas.width / 2, canvas.height / 2 - 1);

// create favicon
let link = document.createElement('link');
link.rel = 'icon';
link.type = 'image/png';
link.href = canvas.toDataURL("image/png");
document.getElementsByTagName('head')[0].appendChild(link);
</script>