Chrome: Refused to display 'http://xenforo.com/community/' in a frame because it set 'X-Frame-Options' to 'SAMEORIGIN'.
FireFox: Load denied by X-Frame-Options: http://xenforo.com/community/ does not permit cross-origin framing.
I always test on my actual device or a reduced browser window. The only way to be truly sure is to be on the platform with the browsers you want to test. Obviously its much easier to test resolution based problems with a desktop browser.
That second site doesn't even have correct resolutions for the devices it portrays making it more of a try out a random low res frame than anything. HTC One is a 1080P device is it not? I mean you have to account for text scaling etc not sure if thats what they are attemping.