You can also make an actually useful and readable SVG favicon this way:
<link
rel="shortcut icon"
href='data:image/svg+xml,%3csvg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">%3ccircle cx="25" cy="50" r="20"/>%3ccircle cx="75" cy="50" r="20"/>%3c/svg>'
/>
Good to know! My goal is simply to stop a 404 popping up during development in the simplest way possible, so the smallest amount of code is best for me.