Hosting the primer with GitHub pages seems to have broken the resolution of the namespace. http://rs.tdwg.org/UBIF/2005/Schema/1.0/SDD.xsd redirects to https://sdd.tdwg.org/2005/Schema/1.0/SDD.xsd which would have been provided by the gh-pages branch but now GitHub pages is served from the docs/ directory in the master branch. I suggest moving the contents of docs/ to the repository root and changing the GitHub Pages configuration to serve there. That should restore access to the items in the official namespace.
Hosting the primer with GitHub pages seems to have broken the resolution of the namespace. http://rs.tdwg.org/UBIF/2005/Schema/1.0/SDD.xsd redirects to https://sdd.tdwg.org/2005/Schema/1.0/SDD.xsd which would have been provided by the
gh-pagesbranch but now GitHub pages is served from thedocs/directory in themasterbranch. I suggest moving the contents ofdocs/to the repository root and changing the GitHub Pages configuration to serve there. That should restore access to the items in the official namespace.