I like the concept! But I believe to make it work better, the repetition should not just of the same puzzle but of the same theme/patter (last rank attack, fork, overloading etc…). Yes, the strict time constraint seems to work for me, but 15 seconds is too short for me to verify any idea, thus I could only react instinctively, which is quite bad in chess.<p>There are some bugs in the program, for example you said on loop 6, the time should be 30 s, but it was only 15 and the loop count was still 1. And it definitely needs a saving/synching feature because even now, one will lose his status just by refreshing the browser.