As much as I love offline web development editors and IDEs, I hate the inconveniences which come with having to edit code, then open a browser to test changes. Even when a browser tab with the test file is open, you constantly have to jump from editor to browser and refresh the tab before any changes will show in the output. I have the preference of working on sites where the output is shown in real-time.
As soon as the code is written, the test-ready output is delivered within seconds. Conveniently, editable code and testable output are shown side-by-side. Another joy of these online playgrounds is the huge community of web designers and developers publishing code snippets for the world to take a look at.