I see no need for "z-index"es or "position: absolute" in your code at all -- unless you have other complications that you have not revealed to us.
To center the background on the DIV class="main":
body{margin:0;padding:20px 0;}
.main{background:transparent url(bg1.jpg) no-repeat center top;}
.content{border:#000 thin solid;width:960px;margin-left:auto;margin-right:auto;background-color:#000;opacity: 0.5;filter:alpha(opacity=50);-moz-opacity: 0.5;}
The "center top" places the center-top of the background image on the center-top of the element it's applied to. You may want to apply a
min-width:1024px;_width:1024px;
to the same element -- to prevent a narrower window from hiding the edges (this will change how the element is rendered if the "viewport" is narrower than the background's dimensions).
The only thing your pre-modified code it can do that my modified code can't:
- Crop the background image (if it is not natively 1024px x 768px) by using the css "width" and "height" properties
- If the class="main" element already has a background image set, most browsers don't support the CSS3 required to stack multiple backgrounds on the same element
Some of what was stated about "z-indexing" and the "position" property above was correct but failed to mention:
you've taken your class="content" element out of "the flow". The ancestor elements won't grow when the content of class="content" element grows. This is an important and fundamental difference between "z-index"ed elements and elements that remain "in the flow".
Other side notes:
- elements with the same z-index are stacked according to their order in the HTML (later in the HTML means they are drawn above on the screen)
- "z-index"ing requires "position: (absolute|relative)", "z-index: (valid value)", and IIRC "(top|left|bottom|right): (valid value)" to take the element "out of the flow"