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A general description is here.
The HTML help system has several new features: LaTeX-like math can be typeset using either KaTeX or MathJax, usage and example code are highlighted using Prism, and for dynamic help the output of examples and demos can be shown within the browser if the ‘knitr’ package is installed. These features can be disabled by setting the environment variable ‘_R_HELP_ENABLE_ENHANCED_HTML_’ to a false value.
‘R CMD check’ no longer runs test scripts generated from corresponding ‘.Rin’ files twice and now signals an ERROR if processing an ‘.Rin’ script fails.