I have a lightbox-style div with scrolling content that I am trying to restrict to a reasonable size within the viewport. I also want this div to be horizontally centered. This is all easy in Fx/Chrome/IE9.

My problem is that IE8 ignores the absolute positioning which I use to size the content, and the rule margin: 0 auto which I use to horizontally center the lightbox.

1) Why? 2) What are my options for workarounds?

EDIT: The centering issue is fixed by setting text-align:center on the parent element, but I have no idea why that works since the element I want to center is not inline. Still stuck on the absolute positioning stuff.

HTML:

<div class="bg">
  <div class="a">
    <div class="aa">titlebar</div>
    <div class="b">
      <!-- many lines of content here -->
    </div>
  </div>
</div>

CSS:

body { overflow: hidden; height: 100%; margin: 0; padding: 0; }
/* IE8 needs ruleset above */

.bg {
 background: #333;
  position: fixed;
  top: 0; right: 0; bottom: 0; left: 0;
  height: 100%; /* needed in IE8 or the bg will only be as tall as the lightbox */
}

.a {
 background: #eee; border: 3px solid #000;
  height: 80%; max-height: 800px; min-height: 200px;
  margin: 0 auto; 
  position: relative;
  width: 80%; min-width: 200px; max-width: 800px;
}

.aa { 
  background: lightblue;
  height: 28px; line-height: 28px; 
  text-align: center;
}

.b { 
  background: coral;
  overflow: auto;
  padding: 20px;
  position: absolute;
  top: 30px; right: 0; bottom: 0; left: 0;
}

Here's a demo of the problem: http://jsbin.com/urikoj/1/edit