I've modified your css:
body {
width: 90%;
margin: auto;
font-family: Arial, Verdana, sans-serif;
font-size: 12px;
background: #e6f0ff;
}
header.beta {
position: absolute;
top: 0;
left: 0;
width: auto;
background: #fff7e0;
opacity: 0.8;
padding: 10px 50px;
color: #444443;
}
header.logo {
width: 90%;
height: 125px;
margin: auto;
margin-top: 60px;
margin-bottom: 30px;
border: 1px dotted grey;
}
nav.tabs {
display: none;
}
div.presentation, div.login, div.categories {
position: relative;
margin-top: 10px;
display: inline-block;
}
div.right {
position: relative;
float: right;
width: 25%;
}
div.presentation {
width: 70%;
clear: both;
}
div.login, div.categories {
width: 100%;
clear: both;
}
div.login header, div.presentation header, div.categories header {
font-size: 14px;
background: #0a97e2;
padding: 3px 15px;
color: white;
border-radius: 10px 10px 0 0;
}
div.login section, div.presentation section, div.categories section {
/*position: absolute;*/
width: 100%;
background: white;
border-radius: 0 0 5px 5px;
}
div.login section p, div.presentation section p {
/* changed this */
margin: 0px 10px;
padding: 3px 10px 3px 10px;
}
div.login section input {
display: block;
margin: auto;
margin-bottom: 10px;
}
/* added this */
div.categories ul {
margin: 0px 10px;
padding: 3px 10px 3px 10px;
}