[JyrkiAlakuijala]

I designed the WebP lossless format, wrote the spec, and implemented the first encoder.

The constraint was in WebP lossy to facilitate exact compatibility with VP8 specification and hoping that it would allow hardware decoding and encoding of WebP images using VP8 hardware.

Hardware encoding and decoding were never used, but the limitation stuck.

There was no serious plan to do hardware lossless, but the constraint was copied for "reducing confusion".

I didn't and don't like it that much as more PNG images couldn't be represented as WebP lossless as a result of that.

[chungy]

Wow, that really sucks. I appreciate the explanation as well as your frustration with it.

My desktop had a mixture of PNG and WebP files solely because of this limitation. I use the past tense because they've now all been converted to JPEG XL lossless.

[JyrkiAlakuijala]

Another group of incompatibility with PNG was 16 bit coding. I had a plan to add it as simply sending two 8 bit images where the second image containing the 8 least significant bits would be predicted to be the same as the first. That way it would not be perfect, but it would be 100x better than how PNG deals with it. WebP had another plan for layers and tiles that never realized, and as a consequence WebP is stuck at 8 bits.