JsFiddle - Online Editor for the Web

Either I don't understand the point of this, or there's a bug in it. I went to the HTML window, entered <p>Hello, world</p> and then pressed [Run]. In the Result window, I got back:

   {"error": "Please use POST request"}

Is that what it should do?

I want to be able to see the automagic stuff in the windows, like the doctype and the javascript framework inclusion. I feel disoriented without it, and I don't think I trust the software to do it the way I want it done -- being able to see it on screen would make me more comfortable.

Why does it do a server round trip every time you tell it to run something?

What are some common examples where using a platform like this would be more efficient that doing in on a common HTML page elsewhere? Regardless, it's a beautiful little system.