I thought I had the same issue, until I realized that only certain lines are editable. On the initial page, only lines 13-16 allow editing, for instance.
It requires JavaScript to run. It works for me in all browsers. Try editing the text on the white background. If you still have trouble you can contact me here: https://www.ever.io/
Me neither (and I don't have JS blocked or anything). Still can run the code though.
Are you editing the code on the white background? Only certain sections are editable.