Modern JavaScript implementations include APIs offering strong cryptography, but it is easy for non-expert developers to misuse them and introduce potentially critical security bugs. In this paper, we formalize a mechanism to rule out such bugs through runtime enforcement of cryptographic API specifications. In particular, we construct a dynamic variant of Security Annotations, which represent security properties of values via type-like information. We formalize Security Annotations within an existing JavaScript semantics and mechanize it to obtain a reference interpreter for JavaScript with embedded Security Annotations. We provide a specification for a fragment of the W3C WebCrypto standard and demonstrate how this specification can reveal security vulnerabilities in JavaScript code with the help of a case study. We define a notion of safety with respect to Security Annotations and extend this to security guarantees for individual programs.