Sizing embedded divs - css

Question

I am trying to emulate a sort of pop-up help dialog for my web site.
When the user clicks help, a mask covers the whole page with a dark, partially transparent mask, and a helppage div with a higher z-order is made visible.
The helppage div is 80% wide and 90% high positioned absolute at left 10% and top 5%, all relative to body.
So far all is great.
The content of the helppage div is:
- a full-width header with a close anchor 20px high floating right.
- a iframe-div to occupy the rest of the helppage div containing:
- an iframe to display the html document in question
The problem:
I would expect the height of the iframe-div to be 20 px less than the helppage div, but for some odd reason it is 3px larger than the helppage div.
As a result the bottom of the iframe is invisible.
The html:
<div id="helpbox">
<div id="helppage" class="window" style="display: block; position: absolute;">
<div class="hd-header">
<a class="close" onclick="hidehelp()"></a>
</div>
<div class="iframe-div">
<iframe id="HelpPageFrame" src="/help-system.html"></iframe>
</div>
</div>
The css:
#helpbox .window {
position:absolute;
display:none;
z-index:9999;
}
#helpbox #helppage {
background: white;
width:80%;
left: 10%;
top: 5%;
height:90%;
padding: 0px;
overflow: hidden;
}
#helppage iframe {
border: none;
width: 100%;
height: 100%;
}
#helppage .iframe-div {
width: 100%;
position: relative;
display: inline-block;
}
#helpbox .hd-header {
height: 20px;
font-weight: bold;
overflow: hidden;
display: inline-block;
width: 100%;
}
#helpbox .close {
width:20px;
height:20px;
display:block;
float:right;
clear:right;
background:transparent url(images/close_icon_double.png) 0 0 no-repeat;
}
Any suggestions would be greatly appreciated
edit As mixel pointed out, an important detail slipped, when I trie to simplify the scenario, that has been corrected.

Answer 1

Please, be accurate, when you are asking questions.
There is whitespace in '#helppage .window' selector. It selects nothing. Because of that '#helppage' is not absolute positioned.
There is no '#helpbox' element.
edit
Though you are still a bit innaccurate (you forgot to close 'DIV'), there is answer. You need to fill the remainder of '#helppage' with '.iframe-div'. If you set '.iframe-div' height to 100%, it takes 100% height of parent element - '#helppage'. To solve this problem you need absolute positioning for '.iframe-div':
#helppage .iframe-div {
position: absolute;
top: 20px;
bottom: 0;
right: 0;
left: 0;
}
Or set height with javascript.
Check out this: Make DIV fill remainder of page vertically?
It's fairly common question.