Question: I have a single background for my site fixed to the top center. I have some blank s that, on mouseover, should toggle the display of an absolutely positioned with a different background that should line up against the original background.

The functionality all works fine; however, the with the background doesn't always line up. It varies depending on the size of the screen by 1px.

This is happening in Chrome, but not Dreamweaver. I haven't tested other browsers.

Any ideas?

Answer: 
The problem was in fact a subpixel rendering issue. I just changed the element's css from "left: 0px;" to "left: -0.5px;" and everything works fine.

Thanks to Mike Robinson for his comment and link: <http://css3wizardry.com/2010/10/05/subpixel-rendering-2/>
Dreamweaver's Live view is a joke, don't trust it for anything other than a rough preview, particularly single pixel differences. Test in the major browsers and use a CSS reset or normalize.css before applying your own styling if things like single pixel differences are an issue for you.

CSS Reset - Resets all styling to 0 so all browsers will start with the same baseline styles. The downside is you have to style every element you intend to use, things like lists will no longer have default styling and appear as plain text. <http://meyerweb.com/eric/tools/css/reset/>

Normalize.css - This is a little more user friendly than a reset, it gives all the base elements the same styling so that all the major browser defaults are covered and rendered the same. <http://necolas.github.com/normalize.css/>