• Build alternative reading experiences without reverse-engineering Author. • Know where semantic and interaction metadata live. • Implement the profile’s precedence rules consistently when the same fact has derived representations. • Safely ignore unsupported extensions. • Implement selected enhancements while retaining ordinary EPUB reading. • Build 2D maps, XR environments, accessibility readers, and analytical readers. • Create independent validators and writers. • Test against a public conformance corpus. • Innovate without inventing another incompatible document format.