No, not idempotent operations; safe operations. When speaking of HTTP methods, PUT and DELETE are both idempotent but not safe; you wouldn’t the user agent to DELETE a resource based on merely a hint—it needs to wait until the actual response comes in, at which point it can confirm whether the header is there and to be acted upon.

See https://tools.ietf.org/html/rfc7231#section-4.2 for the definitions.

(Keep in mind with 103’s semantics that we’re not talking about HTTP methods, we’re merely borrowing its definition of “safe”. Speculative JavaScript execution, for example, is perfectly feasible; you’d simply have to pause execution as soon as it tried to mutate its environment or make HTTP requests or things like that. How useful it is is another question.)