Markdown Runner is a free Chrome add-on that significantly enhances the interaction with Markdown code blocks found on various webpages, including blogs and documentation. It allows users to execute JavaScript, TypeScript, HTML, and CSS directly within the browser, offering features such as auto-generated HTML previews for CSS and sandboxed execution to ensure safety. Additionally, it provides convenient tools like smart copying of clean code, code comparison, and automatic language detection for over 40 programming languages.

The add-on also includes unique functionalities such as code folding for lengthy blocks, an interactive JSON formatter, and the ability to render Mermaid diagrams. Users can customize their experience by toggling features, adjusting settings for code execution, and switching between light and dark themes. With a focus on privacy, Markdown Runner processes everything locally, ensuring that no data is sent to external servers.