I have however noticed an issue with the interactive diagram. When I tried palying with the queens, the AI took my queen en passant, I assume that is not intentional.
The rules say the Queens cannot pass through check. So if you stuck to the rules nothing could have captured you en passant. The capture is the diagram's way to enforce that rule, just as it would capture the royal in the normal way when you illegally step into check, or move away a pinned piece. It captures the King, and declares "I win!".
I did not test how well balanced it is; I could not find a way to easily modify Fairy-Max to support the 'not-passin-through check' rule.
The rules say the Queens cannot pass through check. So if you stuck to the rules nothing could have captured you en passant. The capture is the diagram's way to enforce that rule, just as it would capture the royal in the normal way when you illegally step into check, or move away a pinned piece. It captures the King, and declares "I win!".
I did not test how well balanced it is; I could not find a way to easily modify Fairy-Max to support the 'not-passin-through check' rule.