Whether a reading system has to support a feature is mentioned at the beginning of its section. To be + conformant with this specification, reading systems MUST support all required features as well as all + applicable conditionally-required features (e.g., to support image rendering if the reading system has a + [=viewport=]) as defined in their respective sections.
+ +When supporting recommended and optional features, reading systems MUST meet all normative requirements + as defined in their respective sections.
+ +When reading system developers opt not to support a recommended or optional feature, it does not always + mean none of the normative requirements of the section apply. In some cases, there may be alternative + requirements when not implementing a feature (e.g., to process + fallbacks when scripting is not supported). Reading systems MUST meet these alternative + requirements when not supporting a feature.
+ +EPUB publications frequently contain information not required by this specification (e.g., [=package + document=] metadata). Reading systems may use this additional information for any purposes (e.g., to + improve the user interface).
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