Can You Use a WebP Favicon?
You can reference a WebP image as a favicon and modern browsers will display it, but support isn't universal and nothing that requests /favicon.ico by path will accept WebP. Since PNG and ICO are understood everywhere and favicons are tiny anyway, WebP's compression saves almost nothing, so PNG plus an .ico remains the safe, compatible choice.
It mostly works, but the gain is tiny
Point a rel="icon" link at a .webp file and current browsers render it. The problem is the implicit request: browsers, crawlers, and feed readers hit /favicon.ico by path, and that response has to be an ICO or PNG, never WebP.
WebP's whole appeal is smaller files, but a 32-pixel icon is already a few hundred bytes. There's essentially nothing to compress away, so you take on a compatibility risk for a saving you can't measure.
WebP favicon FAQ
Can a favicon be WebP?
Yes, via a link tag in browsers that support WebP, but not for the /favicon.ico path, which needs ICO or PNG. So WebP can only ever be an extra, not your only icon.
Do browsers support WebP favicons?
Most modern browsers do, but support isn't universal and there's no fallback for clients that request the icon by path, which is why WebP isn't a safe sole format.
Should I use WebP for my favicon?
There's little reason to. Favicons are already tiny, so WebP saves negligible bytes while adding compatibility risk. PNG and ICO are simpler and work everywhere.