IE has traditionally drawn a 2-pixel border around the content area of a site. This border, drawn as part of the page rather than IE’s frame, affects calculations of distance from the top and left of the page. It also creates a not-so-modern beveled look.
In the fourth Platform Preview, you’ll notice pages running in IE9’s Standards Mode no longer have the border. Here’s a before and after:
Pages that run in legacy document modes will still have a 2-pixel border so that any site calculations dependent on the 2 pixels remain the same as in IE8.
To make sure your site runs in IE9 Standards Mode and gets this and all the other latest features in IE9, use a strict doctype. We recommend the HTML5 doctype (<!DOCTYPE html>) since it’s simple and will put your site in Standards Mode in all current browsers.