Simple and sleek! Well designed. I thought it was maybe a bit too simple at first but the puzzles quickly became more interesting/tougher to solve. I only played through the first 4 groups.
I think the game could benefit from having the player hold left mouse to trace the letters. Sometimes I would accidentally hit the wrong letter while trying to navigate my cursor to my desired first letter.
Also possibly an option to rotate the board because reading letters upside down is a little tricky (perhaps with the scroll wheel!)
Good entry overall, well done!