OpenDFDL is a site with various materials related to the Data Format Description Language (DFDL), an emerging standard from the Open Grid Forum.
The github site itself where you find the various repositories is
There used to be a workgroup preliminary draft of the DFDL specification here at this site. This is no longer here.
You can find DFDL working group drafts here:
https://opengridforum.github.io/DFDL
Or the full github site for the OGF DFDL workgroup is here: