Copying my response from the GitHub ticket:
We intentionally do not let HTML5 games embedded on mobile start automatically. Our mobile runner for HTML5 games always makes the game fullscreen since viewport size is limited, and we don’t want people trying to play a game with it scrolling around. Since all games run on fullscreen, we can not start the game immediately since we need to initially show the user the page. Additionally, they can decide whether or not they want to start the project since many html5 games can have high resource & bandwidth costs.
Hope that explains.