You can also open the CSS panel and try changing things yourself. The window update can be viewed live. JS Bin is an open source project that is available on GitHub with instructions on how to host it yourself. Developers can also get a custom skin and starting templates in case they are in the running workshops or are teaching to a much wider audience.
This is a syntax validator that checks for mistakes as well as editors. Unlike a typical code validator, this syntax validator does not bother about coding styles as well as formatting. For syntax errors, this sign will be shown in the left-side gutter of the editor screen. You can place the mouse cursor over that sign, so that it will bring up the error description in detail.