CommonMark with GitHub-Flavored Markdown extensions: fenced code blocks, tables, task lists, strikethrough, and autolinks. Pure CommonMark parsers won't render GFM tables, so the preview may show slightly more than a renderer like Python-Markdown without extensions enabled.

Yes — raw HTML is passed through, which is how GFM handles it too. Note that when you embed Markdown inside HTML, most parsers stop treating it as Markdown on the inside; wrap the Markdown block in a line break to re-activate parsing.

Not in this previewer. GitHub renders Mermaid and KaTeX with special fenced blocks, but that's a GitHub extension, not core Markdown. For math, prefix with $$ and render with KaTeX in your production site. For diagrams, embed a pre-rendered image.

No — the preview runs entirely client-side. Private READMEs, internal docs, and draft blog posts never leave your browser. That matters when your content references unshipped product code or unreleased dates.

Markdown treats two blank lines as ending the list. One blank line between items is a loose list (each item gets <p> wrapping), zero is a tight list (bare text). If you need a paragraph inside an item, indent it 2–4 spaces under the bullet.