Play. This tab is seat 1. When the match exists, a Seat 2 button opens that seat, and that page has a Seat 1 button. Each tab moves only its own seat. The server assigns sequence numbers and closes ticks. Each browser folds the log and draws.
Lobby. Host a match in the first tab (seat 1). In the second tab, type the room code into Join and click Join (seat 2). Once the other player is in the room, a button opens their seat and carries the room code. A room link without a seat only fills the box. The server only introduces the tabs. The host tab runs the match. There is no TURN relay.