Similar to how the language specification was released for PHP a little while back, the HHVM team has announced a new specification for Hack, the language they've created as a part of the HipHop VM project that's similar to PHP.
When we announced Hack, we were very excited for the community to get their hands on a programming language that has helped Facebook engineers become more productive in their day-to-day development and became, alongside PHP, the language used when developing applications running on HHVM. At the time of release, we had documentation geared for the programmer using Hack to develop applications. However, we did not have official documentation for those that might want to create a Hack implementation of their own or something like a Hack conformance test-suite. This specification fills that gap. It is the document for the Hack implementer, and an excellent supplemental document for the Hack user.
The remainder of the post talks about some of the reasoning behind creating the specification, pointing to resources where you can help contribute and a few thanks to some of the people that worked on it.