HTML validation results for the package HTML refmans using the Nu HTML
checker (nu), see <https://validator.github.io/validator/>.

To reproduce, use something like

  install.packages("W3CMarkupValidator")
  install.packages("vnu.jar", type = "source",
                   repos = "https://datacube.wu.ac.at/")
  src <- "/path/to/PKG_VER.tar.gz"   # or just PKG if installed
  out <- "PKG.html"
  tools::pkg2HTML(src, out)
  use("W3CMarkupValidator")
  jar <- system.file("java", "vnu.jar", package = "vnu.jar")
  w3c_markup_validate(file = out, jar = jar)