const input = document.createElement("input");
    document.body.append(input);
    input.type = "file";
    input.accept = "image/*";
    input.onchange = e => {
        if (input.files && input.files[0]) {
            const canvas = document.getElementById("draw-canvas");
            const ctx = canvas.getContext("2d");
            createImageBitmap(input.files[0]).then(img => ctx.drawImage(img, 0, 0));
        }
    };

I’m browsing hn on my iphone, clicked this link, drew a fish with my finger, and saw it

Worked out the box didn't think twice of it

Good point though as the site doesnt follow responsive design patterns

I drew it with the mouse, doable bit you can use your ipad

You could also just load up the website on an iPad with an Apple Pen.