It's surprising that this isn't in a "newer" part of the image format; Huffman compression has been around for over 70 years, Canonical Huffman for a few decades less, but even JPEG uses Huffman. This is a decades-old technology that should've had the bugs worked out of its many implementations by now and there are also countless articles about how to implement it.

I've read the JPEG spec (and written a decoder) before, so I decided to look at the WebP spec: https://developers.google.com/speed/webp/docs/webp_lossless_...

The important information is in section 6. The first thing I notice is that it's not very clear how the codes are constructed, unlike the JPEG one (which actually has a ton of very readable flowcharts on the process), but it appears to be similar to LZH/deflate(zlib). The "specification" looks more like a selected set of source code fragments with accompanying descriptions.

Perhaps I should try writing a WebP decoder too, having already done GIF, JPEG, and PNG, but based on the above "specification", it's almost as if they don't want you to.