Use native Web API identity for ResizeObserver and IntersectionObserver targets and roots, accepting genuine cross-realm objects and registered native proxies while rejecting forged, inherited and author-Proxy values. Validate root membership during dictionary conversion, before later getters, and resolve native targets without reading public nodeType properties.
A 204-case regression matrix covers both observer interfaces and realms, native proxy acceptance, invalid targets, poisoned properties, option conversion order and original exception identity.
Expose the backing-object lookup from the existing native brand layer rather than unwrapping author Proxies in DOM callbacks. Extend its existing identity tests for cross-realm objects, forged objects, registered native Proxies, outer author Proxies and revocation.