Markdown -> HTML fragment, rendered at build time so generated pages carry their content as plain HTML (readable without JavaScript: crawlers, link previews, reader mode, curl). GitHub-flavored, matching what the sites rendered client-side with marked before: tables, strikethrough, task lists, bare URL/www/email autolinks, and raw HTML passed through. Fenced code keeps its language as class="language-x" so highlight.js colors it at runtime. Backed by the vendored md4c (core/ext/md4c).