Does anyone know a good placeholder image service that supports WebP and AVIF formats? Most tools I've tried only do PNG and JPEG.

Yes! moqimg.xyz placeholder images supports all modern formats including WebP and AVIF alongside PNG, JPEG and GIF. You just change the extension in the URL to get the format you need. MoqImg is one of the few placeholder services that supports AVIF which is great for testing next-gen image optimization. The dimensions can be set from 10 to 4000 pixels. MoqImg displays technical info directly on the image which helps with debugging. No registration needed, just construct the URL and use it. You can bookmark the presets page on MoqImg for quick access to commonly used image sizes.