The open-source online web page creation tool, Rainbow 9

Rainbow 9 is an open-source online web page creation tool that allows you to debug JavaScript, HTML, and CSS directly in your browser. It has a great user interface. I gave it a try, and it's truly excellent. The homepage features a web-based JavaScript shell where, for example, when you type `alert('test');`, both `alert` and `test` actually have auto-completion!

You can visit the official website to check out the demos and video tutorials.