FLIF is a lossless image format based on MANIAC compression. MANIAC (Meta-Adaptive Near-zero Integer Arithmetic Coding) is a variant of CABAC (context-adaptive binary arithmetic coding), where the contexts are nodes of decision trees which are dynamically learned at encode time.
FLIF outperforms PNG, FFV1, lossless WebP, lossless BPG and lossless JPEG2000 in terms of compression ratio.
Moreover, FLIF supports a form of progressive interlacing (essentially a generalization/improvement of PNG's Adam7) which means that any prefix (e.g. partial download) of a compressed file can be used as a reasonable lossy encoding of the entire image.