I like the design it's simple maybe create a page about how to use the site cause it's confusing I have no idea what buttons do what and the admin link should be hidden because it could allow some one to try and hack in to the admin panel. The logo shouldn't link to another site it should link to your home page I like that it's responsive and able to be used on a mobile phone
I think you need to develop your own style... The windows design in the middle looks stolen ^^ and for the music function, i would develop a toolbar with a proper design.
You might want to add a real 'about' site (Put 'what is RBLX' on the main page)... and the copyright bar on the bottom belongs to the bottom.
I see though that you put some serious work into it!
Holy Grail? That's from 2006, when we were still trying to support IE5 (though, thankfully, NS 4.x was finally sleeping with the fishes), monitors were 1024 by 768 for the most part, and CSS2 still had spotty browser implementation. The sentiment is more-or-less right, but there are far better ways of doing it now. And there are many good arguments to be made for not filling the screen (unless the screen in question is smallish); line-length has both readability and accessibility implications, image sizes affect both download speed and bandwidth usage (we're still not at a point where we can automatically assume unlimited broadband unless we happen to be running a site that depends on it for basic functionality, like Netflix), and so forth. Pseudo-page-filling is fine (a page that looks big "above the fold", but which uses patterns and solids to get there), as are pages that naturally tile (floats and flexbox can help a lot there) - but even that will cause usability problems on something like a 21:9 monitor unless you deliberately limit the width.