Earlier quoted context omitted.
Ouch. We have come to the point where rendering a favicon requires a full web browser rendering engine.
I can't help but think this is a huge step backwards.
SVG can include foreign objects, which is decidedly useful in some contexts; in browsers, that includes HTML, because it’s easy.
A favicon is an image, so it should allow all the usual types of images, which includes SVG. No reason to exclude SVG or any SVG features other than scripting.
(“But now a non-browser can’t render the favicon properly!” you may say. By using , you’re opting into non-standard functionality and shouldn’t expect all SVG renderers to be able to render that part. For completeness, I’ll note that you can provide an alternative representation for engines that don’t support XHTML in SVG with and requiredExtensions.)