They've already specified three different codec mappings - JPEG, AVC, and HEVC - so to open all HEIF files, you presumably need to implement all of the decoders

They've already specified three different codec mappings - JPEG, AVC, and HEVC - so to open all HEIF files, you presumably need to implement all of the decoders. Worse, there is a financial incentive to implement only a subset of the decoders - HEVC is more expensive than AVC which is more expensive than JPEG. Even things like 4:4:4 color are more expensive than 4:2:0 color for HEVC.