Found the following (possibly) invalid URLs:
URL: http://scikit-learn.org (moved to https://scikit-learn.org/)
From: inst/doc/getting_started.html
README.md
Status: 200
Message: OK
URL: http://stanford.edu/%7Eboyd/papers/pogs.html (moved to
https://stanford.edu/~boyd/papers/pogs.html)
From: inst/doc/getting_started.html
Status: 200
Message: OK
Please change http --> https, add trailing slashes, or follow moved
content as appropriate.
URL:
https://cran.r-project.org/web/packages/h2o4gpu/vignettes/getting_started.html
From: README.md
Status: 200
Message: OK
CRAN URL not in canonical form
The canonical URL of the CRAN page for a package is
https://CRAN.R-project.org/package=pkgname