I recently designed a site that was self-contained within 950px by 500px. Looked real good as a photoshop template. Converting it to html however, was a bitch. Turns out, there's native support for horizontal centering, but no code whatsover to align the entire site vertically central!
And then, it hit me. Flash. Hah! You see, Macromedia (now Adobe) Flash has this option where you can export your file directly as an html page containing the swf. And you can center-align the file! Awesome! So all I had to do was replace that code with mine, and I had a real easy solution to the problem!
1) Make a document in flash, any size.
2) Go to Publish Settings, check the html option.
3) Head over to the new html tab. Keep the "flash alignment: vertical and horizontal" options to center & center.
4) Publish the file. You'll get an html page, where the swf is centrally aligned, and a .js file.