Is AVIF actually smaller than WEBP?

Usually, by roughly 20 to 30 percent at matched quality, though it varies with the image. Photographs gain the most; flat graphics gain the least.

Why can this not be done in my browser?

Browsers can display AVIF but none can currently encode it from a canvas. That is why this conversion runs on the server, unlike the JPG, PNG and WEBP converters.

What if AVIF cannot be produced here?

The page states that AVIF conversion is unavailable on this server and offers the formats that do work. It never quietly hands you a different format and calls it AVIF.

ToolDuck Free online tools

Image Tools

WEBP to AVIF

WEBP to AVIF — the same picture in fewer bytes.

Runs in your browser
Free, no signup