I managed to mess up on the first tutorial level by spawing the block above my head, out of reach. Waited too long, so rewinding did nothing. Had to hard-reload the page to retry (might want to have a restart option).
After retrying, I messed up again in the next level; this time I tried to rewind time, but the blocks did not come back in, so I'm not really sure what the rewind is used for (might be because it's introduced at a later level).
I really liked the audio, it was very well done, and the graphics were cool too.