What exactly are these “intrinsic APIs”? They’re the APIs defined in the ECMAScript Language Specification. That is, things built into JavaScript. This includes Annex B of the specification, which deals with legacy browser features that aren’t part of the “core” of JavaScript, but are nevertheless still supported for JavaScript running inside a web browser.
Note that some features of JavaScript that you might consider intrinsic—for example, the window and document global variables—are superceded by SecureObject objects, which offer a more constrained API.
This rule verifies that use of the intrinsic JavaScript APIs is according to the published specification. The use of non-standard, deprecated, and removed language features is disallowed.