This is something I've struggled with before and I still don't know how to resolve it.
My game is published on Clay.io, here Korelos
It works fine there and is centred correctly on the screen. The screen resolution is 800 x 600 which is the maximum size for some sites such as Kongregate.
But when I link that frame to Kongregate, it works but there are scrollbars around the edge of the game and the whole game isn't showing (kind of makes it unplayable because you can't click the icons and so on).
Since the game is 800x600 and it's listed as 800x600 on the Kongregate upload, any ideas why this is happening and how I can get rid of the scrollbars?