I am making a layout that I would enjoy to play around with further later.

Currently, this is my HTML.

```
<!DOCTYPE html>
<html>
<head>
    <title>title</title>
<link rel="stylesheet" type="text/css" href="style.css">
</head>

<body>
<div id="top">
    <a class="title" href="index.html">TITLE</a>
</div>
<div id="middle">
    <h2>SubTitle</h2>
</div>
<div id="bottom">
    <ul>
         <a class="nav" href="#"><li>NAV1</li></a>
         <a class="nav" href="#"><li>NAV2</li></a>
         <a class="nav" href="#"><li>NAV3</li></a>
         <a class="nav" href="#"><li>NAV4</li></a>
         <a class="nav" href="#"><li>NAV5</li></a>
    </ul>
</div>
</body>

</html>

```

And CSS.

```
HTML, body {
margin: 0px;
padding: 0px;
height: 100%;
width: 100%;
color: #0d0d0d;
}

#top {
display: block;
width: 100%;
height: 15%;
border-bottom: 2px solid blue;
background-color: #f2f2f2;
box-sizing: border-box;
overflow: hidden;
margin: 0px;
padding: 0px;
}

.title {
display: block;
padding-top: 2%;
width: 100%;
text-align: center;
color: #0d0d0d;
font-family: arial black;
text-decoration: none;
font-size: 4em;
}

#middle {
display: block;
max-width: 600px;
height: 75%;
max-height: 75%;
margin: 0 auto;
}

#bottom {
display: block;
width: 100%;
height: 10%;
background-color: #f2f2f2;
margin: 0px;
padding: 0px;
}

ul
{
list-style-type:none;
margin:0;
padding:0;
width: 100%;
height: 100%;
}

li
{
float: left;
width: 20%;
height: 100%;
text-align: center;
box-sizing: border-box;
border-top: 2px solid blue;
border-right: 2px solid blue;
padding-top: 3%;
overflow: hidden;
-webkit-transition: color 500ms;  
-moz-transition: color 500ms;  
-o-transition: color 500ms;  
transition: color 500ms;   
}

.nav
{
display: inline;
text-decoration: none;
color: #0d0d0d;
font-family: arial black;
}

li:hover
{
color: orange;
}

```

The problem lies in the fact that when I fill the "middle" div with any content, such as headers or paragraphs, its height changes to expand past what it should be. The layout is intended to have no scroll-bar, which is why the the percentages of the div add up to 100%. 

Why does the middle div grow when content is added? How can I fix this? 

Thanks.