Can manual be online, divided in sections and when you finish a level - the game opens page that describes where are you now with needed instructions of player's current state. Is it ok to make that kind of 'interactive' manual?
strictly speaking, having the game open a webpage at certain times means the game is giving you additional feedback. try to think if there's any way to make the game you want to make using a static manual. if there is, great! if there isn't, bending the rules isn't the end of the world!