Keys "s"=save "l"=load "b"=back once
Hans Kluever and Peter Kahl, 1968. This is a very popular variant. I have used the NOST rule that you cannot push or pull your king through check. The "ko" rule about immediately repeating a previous position has been implemented.
There are still some bugs here, especially about seeing check.
These are simple illustrations rather than strong opponents.
Bug reports? -- Thank you! Keep them coming!