I recently finished up a 2 player chess game in Construct for Synaptop (using their API calls to network between 2 users.) You can play the game for free on Synaptop. Looks like I can now link to a page for my game, and not have to rely on just linking to Synaptop's main page.
Some of the functionality I've worked in is allowing users to only move legally, while supporting all special moves in chess, including castling and en passent. I also take into account check and force players to not move into check and move out of check when in check, and calculate check mate and stalemate. (That last part was the hardest part for me to code in in any sort of optimized way, does anyone have an optimized checkmate algorithm for construct events?)
As always, I welcome any questions comments concerns.