This is a standard for files specifying rules for serving content from static files via HTTP(S), as originally formulated in [isaacs/github#408](Initial work at https://github.com/isaacs/github/issues/408). More information on the design and reasoning of the standard can be found in the explainer document.
The working draft for the standard is in the "draft" directory. Right now, files aren't specified in formal, low-level EBNF syntax: they're instead described at the high level, for authors and implementers to understand how to author and implement them, respectively.