I have three divs: head, foot and textbox. 
The head and foot divs are fixed positions, and the third div is partly fixed (margin-top). 

My question is: How can I change the textbox's div bottom to fix different monitors size? I can't use 100% height because it hangs on foot div. In this homepage I don't use scrollbar, because the backgrounk is changing image files. I woud like to make it somehow the margin-bottom part keep distance the monitor's bottom.

---

```
<html>
<head>
<title>Div bottom</title>
<style>
.head{
    position:absolute;
    clear:both;
    top:0px;
    right:0px;
    float:right;
    width:100%;
    height:80px;
    background-color:grey;
}
.foot {
    position:fixed;
    clear:both;
    height:35px;
    right:0px;
    float:right;
    width:100%;
    background-color:grey;
    bottom:0px;
}
.textbox {     
    overflow: hidden;
    position: relative;
    padding:20px;
    border: 1px solid gray; 
    background-color:red;
    z-index:0;
    text-align:justify;
    color:black;
    line-height: 2em;
    border-radius: 3px;
    margin-top:100px;
    width:910px;
    margin-left: auto; 
    margin-right:auto;
}
</style>
</head>

<body>
<div class="head">HEAD</div>

<div class="textbox">?</div>

<div class="foot">FOOT</div>
</body>
</html>

```