Out of curiosity what kind of traffic is safe?

In the context of chess I would imagine passing JSON around via EventSource/SSE should be more than enough.

I'm not sure. The GFW is rather tricky and is known to do DPI.

For now, yes we are communicating moves in exactly the way you described EventSource! Eg:

{ "fen": "rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1", "status": "active", "turn": "b", "white_ready": true, "black_ready": true, "spectators": 1, "in_check": false, "last_move": "e2e4", "game_over": false, "available_colors": [], "reservations": { "white": {"reserved": true, "expires_in": 0}, "black": {"reserved": true, "expires_in": 0} } }