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A general description is here.
Setting environment variable ‘_R_CHECK_RD_VALIDATE_RD2HTML_’ to false value will override ‘R CMD check --as-cran’ and turn off HTML validation. This provides a way to circumvent a problematic ‘tidy’ such as the 2006 version that ships with macOS.
‘R CMD check’ can optionally (but included in ‘--as-cran’) validate the HTML produced from the packages ‘.Rd’ files. See <URL: https://blog.r-project.org/2022/04/08/enhancements-to-html-documentation/index.html> : this needs a fairly recent version of HTML tidy to be available.