Pre-CR, I've just removed this Editorial Note. I assume we should address this during CR when we'll know if explicit salts are supported in terms of interop.

---

Editorial note

The definition of HKDF allows the caller to supply an optional pseudorandom salt value, which is used as the key during the extract phase. If this value is not supplied, an all zero string is used instead. However, support for an explicit salt value is not widely implemented in existing APIs, nor is it required by existing usages of HKDF. Should this be an optional parameter, and if so, what should the behavior be of a user agent that does not support explicit salt values (is it conforming or non-conforming?)