Obviously the tools behind this exercise are broken. The preview window shows an error "Parse error: syntax error, unexpected ';' on line 12"
If the preview window is set to full-screen, the expected output is displayed. Switching out of full-screen mode returns to the unexpected ; message. EVEN if there is no line 12.
The previous CSS is still open in the editor (with the .coin division) Deleting that does not improve the situation.
I see that many people here have written valid while loops, but the editor/.interpreter rejects them, or hangs. I know that getting a simulator like this working in a browser (Chrome in my case) is hard, but it seems that many reports have been submitted and no action has been taken to check them properly.
Admittedly, many of the posts about this section are vague.