Lossless WebP images maintain the original image quality. Lossy WebP images will result in reduced image quality, although it’s unlikely to be noticeable for standard website images.
If your website images need to be high resolution or viewed large on a retina display, you need to check that WebP compression is not negatively affecting your image quality.
You can view a sample of WebP-lossless and WebP-lossy images in Google’s WebP gallery.