Webp is much better for lossless.

I have a similar tool at https://pictolab.io. I use it for tweaking the brightness and satuation of images mostly. For that you want HDR to reduce clipping, and most image formats have surprisingly bad HDR support besides AVIF (and jxl). Jpeg kinda supports it with gain maps but it's kind of tacked-on to the spec. Webp doesn't support HDR at all.

It's also surprisingly hard to support HDR input. For example, Apple uses a weird poorly documented format for the HEIC their iPhones take. I feel like HDR is just kind of a mess right now.