WebP Lossless is a comparably simple codec; it is basically a PNG with more transformation methods and custom entropy coding. So I believe it is probably more doable to rewrite in Wuffs than other codecs. The entirety of WebP has a lot more allocations though, a brief skim through libwebp shows various calls to malloc/calloc for, e.g. I/O buffering (src/dec/io_dec.c among others), alpha plane (src/dec/alpha_dec.c), color map and transformation (src/dec/vp8l_dec.c), gradient smoothing (src/utils/quant_levels_dec_utils.c) and so on.