It doesn't matter too much (for the web) as you can use <picture> and <source> and specify multiple jxl/avif/webp/jpeg sources, and the browser will choose the first type it understands. These elements are widely supported.

Hopefully JXL doesn't become yet another image format (927)... some features of it are incredible. For example, you can have mixed lossless/lossy content in one image. No longer will you need to choose between a lossy or lossless format: the encoder would intelligently segment the image.