I would also second the idea of putting the seed in the URL, even with just an anchor link so that all processing is done in the browser. So if someone visits https://tanck.nl/wallpaper/#12345, use this number as the seed. You could also add a link under the image.

edit: here's a hacky way to do it, since you can't seed Math.random() in JavaScript; add this at the start of the "draw()" function.

    const url = new URL(location.href);
    const match = /#([0-9]+)$/.exec(url.hash);
    if (match) {
        var seed = parseInt(match[1]);
        Math.random = function() {
            var x = Math.sin(seed++) * 10000;
            return x - Math.floor(x);
        };
    }

Based on a StackOverflow answer to the commonly-asked question of how to seed the PRNG in JavaScript, by all means not a perfect solution but appropriately concise for this short script: https://stackoverflow.com/a/19303725