“Capability delegation” means allowing a frame to relinquish its ability to call a restricted JS API (e.g. popups, fullscreen, Payment Request and transfer the ability to another (sub)frame it can trust.
We are proposing a generic model for capability delegation that could be utilized by any JS APIs after they have defined their own “special case” delegation behavior. The primary use-case we want to support is allowing online shopping websites to outsource payment collection/processing to third-party Payment Service Providers (PSPs).
Details of the proposal appears here: