There is Ruby code which compiles the .md sources files into a structure of HTML files. This leverages the commonmarker Ruby gem.
The Ruby code decides how the pages are laid out. CSS is used to vary the way the website looks at different screenwidths - different font sizes and image sizes are used at different resolutions.
The Nu Html Checker is run over the generated HTML and CSS to check for correctness.
The generated HTML and PHP file are spell checked against local word lists.
Local links in the generated HTML and PHP files are checked that they are correct, and go to files inside the generated webpages. This is something that does get broken as the code changes, and also isn't immediately obvious just looking at the webpages.
Some of the generated HTML files are checked against a set of golden files.
rsync is run to sync any changes up to the webspace for the website.