Interesting idea. It combines basic computer science teaching with fun, interactivity, and testing the students.
(Only tried it from my Android mobile till now, and the Run button did not result in any output. Have to try it from a PC.)
Turtle graphics is only one way of giving training on the site, though. Anyone can host a course on the site for free.
I had first programmed turtle graphics in the LOGO programming language on my first home computer, an IBM PC Jr. It was fun. LOGO, a sort of simplified Lisp, was easy to learn and still fairly powerful. It's a good language to teach children programming.
Later, some time after first learning Python, I got to know that Python also has a turtle graphics module, which uses Tkinter, and tried it out.
Runestone, being web-based, probably uses the HTML5 canvas element.
About Runestone Interactive:
Python turtle graphics module:
Logo language on Wikipedia:
Update: For kids (of all ages :-) interested in trying out LOGO, there are some free versions available.
Just do a Google search.