Put whatever you want to center in a box (DIV) and set its width and position. Like this:
Then center the left edge of the container on the page -
Next, move the container left by half its own width -
margin-left: -50%; // or in this case, 400px.
The effect is to move the left edge of the container back by half its own width from the page center, thus exactly centering the page, no matter what width the screen is.
Next put your content in the div -
Your nicely centered content entered in here
This is a proven cross browser method of centering page content.
Here is a shorthand way that only works with the newer browsers.
margin-left; 20px auto;