Yup. My last "golfing" take was to use default font-size and fit viewBox accordingly:
data:text/html;charset=utf-8,<link rel=icon href='data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" viewBox="1,-15,19,19"><text>%F0%9F%A6%84</text></svg>'>
But haven't tested outside Windows, so it will be most probably off in some browsers (Safari?).
Or this funky quine document with same unicorn emoji that rotates or fades depending on prefers-reduced-motion:
data:text/html;charset=utf-8,<!doctype html>%0A<link rel=icon href='data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" viewBox="1,-15,19,19"><text transform-origin="10 -3">%F0%9F%A6%84</text><style>@keyframes x{to{transform:rotate(1turn)}}@keyframes y{to{opacity:0}}text{animation:2s x infinite alternate}@media(prefers-reduced-motion){text{animation-name:y}}</style></svg>'>%0A<title>Animated favicon affected by `prefers-reduced-motion` media query, embedded in datauri inside datauri document</title>%0A<style>:not(style,script){display:block; padding:0 .2em; background:darkslategray; color:snow; white-space:pre-wrap} title::after{content:' (in `' var(--prm) '` mode)'}html{--prm:'no-preference'}@media(prefers-reduced-motion:reduce){html{--prm:'reduce'}}</style>%0A<script>document.write('<plaintext>',decodeURIComponent(document.URL))</script>
