Try IT Yourself HTML5 editor per Francesco De Stefano

This WebExtension provides you with a very easy accessible and offline playground for your lovely web apps and experiments.
Perfect for developers who want to experiment or practice in HTML/CSS/JS rapidly, even without Internet connection.

- Works offline
- Save as HTML file
- You can activate full screen mode for each editor
- You can export your HTML code to PDF
- Code Beautify
- Import & Export all creations anytime, anywhere
- Very easily accessible. Simply open a new tab in Firefox!
- Preview of your web design
- Supports tables for HTML5, CSS3, etc, with "Can I use" APIs