Why did the width collapse in the percentage width child element in an absolutely positioned parent on Internet Explorer 7?

Question

The parent div needs to have a defined width, either in pixels or as a percentage. In Internet Explorer 7, the parent div needs a defined width for child percentage divs to work correctly.

Here is a sample code. I think this is what you are looking for. The following code displays exactly the same in Firefox 3 (mac) and IE7.

#absdiv {
  position: absolute; 
  left: 100px; 
  top: 100px; 
  width: 80%; 
  height: 60%; 
  background: #999;
}

#pctchild {
  width: 60%; 
  height: 40%; 
  background: #CCC;
}

#reldiv {
  position: relative;
  left: 20px;
  top: 20px;
  height: 25px;
  width: 40%;
  background: red;
}

<div id="absdiv">
    <div id="reldiv"></div>
    <div id="pctchild"></div>
</div>

IE prior to 8 has a temporal aspect to its box model that most notably creates a problem with percentage-based widths. In your case here an absolutely positioned div by default has no width. Its width will be worked out based on the pixel width of its content and will be calculated after the contents are rendered. So at the point, IE encounters and renders your relatively positioned div its parent has a width of 0 hence why it itself collapses to 0.

If you would like a more in-depth discussion of this along with lots of working examples, have a gander here.

Why doesn’t the percentage width child in absolutely positioned parent work in IE7?

Because it's Internet Explorer.

Is there something I'm missing here?

That is, to raise your co-workers' / clients' awareness that IE is not good.

Is there an easy fix besides the pixel-based width on the child?

Use em units as they are more useful when creating liquid layouts as you can use them for padding and margins as well as font sizes. So your white space grows and shrinks proportionally to your text if it is resized (which is really what you need). I don't think percentages give a finer control than ems; there's nothing to stop you specifying in hundredths of ems (0.01 em) and the browser will interpret as it sees fit.

Is there an area of the CSS specification that covers this?

None, as far as I remember em's and %'s were intended for font sizes alone back at CSS 1.0.

I think this has something to do with the way the hasLayout property is implemented in the older browser.

Have you tried your code in IE8 to see if works in there, too? IE8 has a Debugger (F12) and can also run in IE7 mode.

The div needs to have a defined width:

<div id="parent" style="width:230px;">
    <div id="child1"></div>
    <div id="child2"></div>
</div>

The parent <div> tag doesn't have any width specified. Use it for the child <div> tags, it could be percentage or pixel, but whatever it would be,it should link to its appropriate positions:

<div id="MainDiv" style="width:60%;">
    <div id="Div1">
        ...
    </div>
    <div id="Div2">
        ...
    </div>
    ...
</div>