toc: true renders a fixed right-hand table of contents that is hidden below a 1700px-wide viewport, so it never overlaps the prose — on a typical laptop you will not see it.

datetime is never generated for you. The colophon shows exactly what frontmatter says, in preview and export alike. Only the copyright year © <year> comes from the current date.

Name sibling files with the locale immediately before the extension:

guide.mdx          # base fallback
guide.zh-CN.mdx    # Simplified Chinese variant
guide.en-US.mdx    # English variant

The active UI language picks its exact variant first, then the unsuffixed base. Navigation shows one logical guide.mdx entry rather than one per physical file. A localized ?doc= URL is accepted and normalized to the active language when that family has a match or a base fallback, and relative Markdown links use the same family-aware routing.

Only the exact .zh-CN and .en-US suffixes are special; anything else is an ordinary filename. mdxx stays a physical-file exporter — it exports the file you pass and never bundles siblings.

• It prepends, it does not replace. Your font goes ahead of the built-in chain, so a missing glyph falls through. A Latin-only face takes over Latin and digits while CJK still falls back to the embedded Source Serif 4 and the system serif — you never write a fallback chain yourself.
• config set never guesses. It merges, keeping your other settings and any key it does not know. If the existing file cannot be understood — invalid JSON, a non-object at the top level — it refuses to write and says so rather than overwriting what it cannot read. Rewriting a commented file does drop the comments, and it tells you that too.
• Both commands honor it, so the preview shows the fonts the export will use. But the export records font names only and embeds no font files: the artifact stays free of external links, and a recipient without the font falls back down the chain. Identical glyphs for everyone would mean embedding the file — licensing and size implications, not supported.
• A broken config never blocks a run. A missing file is the normal case. An unreadable file, invalid JSON, a wrong field type or an illegal font name each print one Warning: line to stderr and fall back to the defaults. Font names allow only letters, digits, spaces and . _ + -; if any name in an entry is illegal the whole entry falls back rather than partly applying.

bin/          mdxv.mjs (preview) · mdxx.mjs (export)
src/
  cli/        input resolution · Vite config · virtual-module plugin · CLI language ·
              localized-doc families · user-level config (fonts) · terminal output
  mdx/        compile plugin list · three-lane diagram rehype plugin
  i18n/       supported locales · message catalog (product strings only)
  app/        React app: Layout · component library · theme.css ·
              MDXProvider mapping · preferences (language / theme)
demo/         index.mdx · index.zh-CN.mdx — the bundled component gallery
examples/     demo.mdx · guide/intro.md
test/         node --test suites (unit / integration / export smoke)
e2e/          Playwright specs + fixtures

Preview starts the Vite dev server programmatically: a single doc loads through the virtual module virtual:mdx-target, while directory mode scans for .md/.mdx, serves /__mdxv/tree, and lets the frontend load by ?doc= and route relative links.

Export runs vite build with vite-plugin-singlefile, inlining every asset — KaTeX fonts and the Mermaid runtime included — into one HTML file with no external references.

test/ uses Node's built-in node --test. Browser behaviour lives in e2e/, driven by Playwright — the only devDependency.

make test          # all node tests (unit + integration + export smoke; no e2e)
make test-unit     # L1: in-process, zero subprocesses (sub-second)
make test-cli      # L2: CLI subprocess contracts, no vite build
make test-build    # L3: everything needing a real vite build (slowest)
make test-e2e      # Playwright (first run: npx playwright install)

• unit — input resolution, localized-doc families, locale and message lookup, CLI language precedence, user config, terminal output. Fixtures built in a temp dir.
• integration — runs mdxOptions() through the official @mdx-js/mdx compile(), asserting frontmatter, GFM, math, highlighting and all three diagram lanes fire.
• export smoke — runs the real mdxx and asserts the output is zero-external-link and inlined.
• e2e — language and theme preferences and their persistence, localized variants, empty states.