The new IDNA/UTS 46 implementation increases implementation independence, considering that previously the three engines all used ICU4X. Both the new UTS 46 layer (not part of ICU4X) and the underlying normalization/mapping layer (part of ICU4X) are novel implementations and not ports of ICU4C […]

… 1) they use 3 ideas from ICU4C that are not apparent from the spec: a) the UTS 46 mapping and the normalization steps are fused into a special normalization, b) STD3 ASCII rules are a post-processing deny list and not in the mapping data, and c) the DoS-avoidance length limits come from […]