"[letterboxing] works by adding white margins to a browser window so that the window is as close as possible to the desired size while users are still in a coup

"[letterboxing] works by adding white margins to a browser window so that the window is as close as possible to the desired size while users are still in a couple of screen size buckets that prevent singling them out with the help of screen dimensions." These margins may be surprising (ie. look like a bug) if you don't know what they are.