Its about quite a few things. You can use CSSDeck to make testcases and share with other people on IRC, forums, StackOverflow, etc. who help you solve your coding problems. You can even code with multiple people in realtime in various modes. Remember etherpad ? Yeah we've got something like that for HTML/CSS/JS coding with lots of other bells and whistles.
If you're creating some cool crazy stuff with the powers of CSS3 and JS you can post it here and we're going to feature it in the gallery helping you get some fame as well as helping others learn about the cutting edge practices and methodologies. More Info »
You can also try creating or watching codecasts which is basically a recording of whatever code is typed in the code editors. This way other people can "playback" your recording in a video style and learn how the creation was developed.
+ JS and CSS Resources
+ Private Bin
+ Super Private Bin
+ Optionally, a custom slug for the URL of your item can also be set :)
- You can select the Prefixfree option to avoid writing vendor-prefixes in your CSS code.
- You can choose Normalize to include Normalize.css
- You can position editors vertically or horizontally from the Preferences Panel.