Div 100% height works on Firefox but not in IE

I have a container div that holds two internal divs; both should take 100% width and 100% height within the container.

I set both internal divs to 100% height. That works fine in Firefox, however in IE the divs do not stretch to 100% height but only the height of the text inside them.

The following is a simplified version of my style sheet.

#container
{
   height: auto;
   width: 100%;
}

#container #mainContentsWrapper
{
   float: left;

   height: 100%;
   width: 70%;
   margin: 0;
   padding: 0;
}


#container #sidebarWrapper
{      
   float: right;

   height: 100%;
   width: 29.7%;
   margin: 0;
   padding: 0;
}

Is there something I am doing wrong? Or any Firefox/IE quirks I am missing out?

I think "works fine in Firefox" is in the Quirks mode rendering only. In the Standard mode rendering, that might not work fine in Firefox too.

percentage depends on "containing block", instead of viewport.

CSS Specification says

The percentage is calculated with respect to the height of the generated box's containing block. If the height of the containing block is not specified explicitly (i.e., it depends on content height), and this element is not absolutely positioned, the value computes to 'auto'.

so

#container { height: auto; }
#container #mainContentsWrapper { height: n%; }
#container #sidebarWrapper { height: n%; }

means

#container { height: auto; }
#container #mainContentsWrapper { height: auto; }
#container #sidebarWrapper { height: auto; }

To stretch to 100% height of viewport, you need to specify the height of the containing block (in this case, it's #container). Moreover, you also need to specify the height to body and html, because initial Containing Block is "UA-dependent".

All you need is...

html, body { height:100%; }
#container { height:100%; }

I'm not sure what problem you are solving, but when I have two side by side containers that need to be the same height, I run a little javascript on page load that finds the maximum height of the two and explicitly sets the other to the same height. It seems to me that height: 100% might just mean "make it the size needed to fully contain the content" when what you really want is "make both the size of the largest content."

Note: you'll need to resize them again if anything happens on the page to change their height -- like a validation summary being made visible or a collapsible menu opening.

Its hard to give you a good answer, without seeing the html that you are actually using.

Are you outputting a doctype / using standards mode rendering? Without actually being able to look into a html repro, that would be my first guess for a html interpretation difference between firefox and internet explorer.