Facebook Facebook Zstandard, or "zstd" (pronounced "zee standard"), is a data compression mechanism. This document describes the mechanism and registers a media type and content encoding to be used when transporting zstd-compressed content via Multipurpose Internet Mail Extensions (MIME). Despite use of the word "standard" as part of its name, readers are advised that this document is not an Internet Standards Track specification; it is being published for informational purposes only.