The repo ships a built dist/ and vendor/, so no build is needed.

1. Open chrome://extensions
2. Enable Developer mode (top right)
3. Load unpacked → select this folder

To read local .md files, the popup walks you through the one Chrome switch it needs ("Allow access to file URLs").

• GitHub-flavored Markdown — tables, task lists, strikethrough, autolinks.
• KaTeX math — $inline$ and $$block$$, plus bare LaTeX environments (\begin{align}…), rendered offline. Double-click any formula to copy its LaTeX.
• Syntax-highlighted code with a copy button on every block.
• Mermaid diagrams — ```mermaid fences render as diagrams, lazy-loaded so pages without them pay nothing.
• YAML frontmatter rendered as a clean header card instead of raw --- text.
• Auto-reload — re-renders in place when the file changes on disk, with fallback transports and a silent failure mode so it never breaks the page.
• Copy for AI — turn the whole rendered document back into clean markdown, ready to paste into your next prompt.
• Table of contents sidebar with scrollspy.
• Light and dark themes, plus a density setting, synced across devices via chrome.storage.sync.
• Reading zoom (opt-in).
• Hebrew/English bidi — each block picks its own text direction for mixed RTL/LTR documents.
• Raw source view and copy-selection-as-markdown.
• Print / export to PDF with print-friendly styling.
• Tab / Shift+Tab to step through the document block by block (and through table rows).
• Folder view — enhances file:// directory listings, plus a Folder button to jump to a file's containing folder.
• Force-render from the popup — turn any page into a rendered markdown view on demand.
• Robust encoding handling — UTF-8/UTF-16 BOM detection, windows-1252/1255 heuristics for undeclared legacy encodings.
• Large-file guard so huge documents don't lock up the tab.

Supported: .md, .markdown, .mdown, .mkd, .mkdn, .mdx, from file:// and from http(s):// when the server sends the file as plain text (most static hosts, including GitHub raw and CDNs, do).

Skim only acts when Chrome is showing a markdown file as plain text (both file:// and most static hosts serve .md as text/plain or text/markdown). A content script:

1. Detects the plaintext markdown page and its encoding (src/detect.js, src/encoding.js).
2. Extracts the raw source, pulls out YAML frontmatter (src/frontmatter.js), parses with marked, renders math with KaTeX, highlights code with highlight.js, and sanitizes the result with DOMPurify (src/render.js, src/table.js).
3. Replaces the page with a styled article and mounts the UI: TOC, toolbar, theme/zoom, copy buttons, print, Copy-for-AI (src/ui.js, src/main.js, src/nav.js, src/print.js, src/copy-markdown.js, src/skim.css).
4. Watches the file for changes and re-renders in place (src/reload.js).

Markdown served as text/html or as a forced download is not intercepted. Mermaid renders from a separate, lazily-loaded bundle (src/mermaid.js, src/mermaid-entry.js). A separate content script (src/folder.js) enhances file:// directory listings. The popup, options, and onboarding pages share a settings module (src/settings.js) backed by chrome.storage.sync.

The repo ships a built dist/ and vendor/, so installing unpacked needs no build. To rebuild after editing src/:

npm install
npm run build            # bundle content scripts + vendor KaTeX/mermaid assets
npm test                 # run the unit tests (node --test)
npm run size             # check the shipped package against its size budget
node scripts/preview.mjs examples/sample.md   # write preview.html to eyeball