I found
this trick, but I think it is limited to browsers and isn’t universally supported.
Code:
<link rel="icon" href="data:image/svg+xml,<svg xmlns=%22http://www.w3.org/2000/svg%22 viewBox=%220 0 100 100%22><text y=%22.9em%22 font-size=%2290%22></text></svg>">