Comment by Tepix

Instead of going via pixels, why not use a SVG favicon and directly store markup inside it and extract it?

Use this favicon.svg:

    <svg xmlns="http://www.w3.org/2000/svg">
    <circle cx="50%" cy="50%" r="50%" fill="orange"/>
    <p>hello HN!</p>
    </svg>

use this in your <head> to use a svg favicon:

    <link id="favicon" rel="icon" href="favicon.svg" type="image/svg+xml">

finally, use this in your <body> to extract it and add it to your document body:

    <script>
    fetch(favicon.href).then(r => r.text()).then(t => document.body.innerHTML += t.match(/<p[\s\S]*p>/)[0]);
    </script>