Actually, the manifest.json requirement turns out to be a bit of a snag. pkgdown::deploy_to_branch() does not generate a manifest automatically, and I thought I could hack one and stick in pkgdown/favicons/, but it's hard to keep it up to date. At the moment, it seems I need to write something else in the last step of this convenient GitHub Actions workflow file. Hoping pkgdown will eventually write its own manifest in deploy_to_branch().