WebP supports lossy and lossless images, including transparency. Current Chrome, Firefox, Safari and Edge can display it. That makes it a practical output for a browser tool.
AVIF often reaches a similar visual result at a lower byte count. Encoding support is uneven. This site offers AVIF only when the current browser reports that it can create an AVIF blob. A page is not published as a dedicated AVIF converter because the result would depend on the visitor's browser.
If a client cannot decode AVIF, keep a JPG or WebP version as well.