WebP is much more limiting than JPEG XL. in lossy mode WebP has forced 4:2:0 chroma subsampling, supports only 8 bit per channel colors (really only about 7.8 bits, because thanks to WebP being tv-range the values aren't in a 0-255 range but in a 16-235 range, causing even more color banding than 8 bit per channel already has), no HDR, a maximum resolution of 16385 x 16385 making it unsuitable for larger images...
JPEG XL on the other hand supports up to 4099 color channels, a bit depth up to 32 bit per channel (technically up to 64 bit, but this currently isn't used), supports HDR natively, can use splines to compress elements like strands of hair, thin tree branches or line art that are typically hard to compress with DCT, supports patches for compressing repeating image elements, supports thermal, depth and alpha channels, supports losslessly recompressing existing JPEGs saving about 20%, supports CMYK and spot colors for printing, supports layers and selection masks, supports storing raw camera sensor data in bayer patterns, etc.
WebP is just a web delivery format, JPEG XL was designed to support many uses cases like web delivery, medical imaging, raw camera sensor data, authoring, multi-spectral imaging... the list goes on. if we support JPEG XL now, chances are it'll be quite a while before we need a new general purpose image format because JPEG XL covers so many current use cases and was designed to accommodate potential future use cases as well.
I didn't realize WebP was limited-RGB in addition to 4:2:0.
According to RFC 9649, this is accurate.
While the ITU-R Recommendation 601 on color is only a "SHOULD" in the RFC, you'd need a custom decoder to break out of limited RGB:
> The VP8 specification describes how to decode the image into Y'CbCr format. To convert to RGB, Recommendation 601 [REC601] SHOULD be used. Applications MAY use another conversion method, but visual results may differ among decoders.
JPEG XL on the other hand supports up to 4099 color channels, a bit depth up to 32 bit per channel (technically up to 64 bit, but this currently isn't used), supports HDR natively, can use splines to compress elements like strands of hair, thin tree branches or line art that are typically hard to compress with DCT, supports patches for compressing repeating image elements, supports thermal, depth and alpha channels, supports losslessly recompressing existing JPEGs saving about 20%, supports CMYK and spot colors for printing, supports layers and selection masks, supports storing raw camera sensor data in bayer patterns, etc.
WebP is just a web delivery format, JPEG XL was designed to support many uses cases like web delivery, medical imaging, raw camera sensor data, authoring, multi-spectral imaging... the list goes on. if we support JPEG XL now, chances are it'll be quite a while before we need a new general purpose image format because JPEG XL covers so many current use cases and was designed to accommodate potential future use cases as well.