body {
background-color: lightgray;
font-family: Arial, Helvetica, sans-serif;
}
#container {
width: 1100px;
margin: 0px auto;
border: 4px solid gray;
}
header {
background-color: red;
height: 100px;
width: 100%;
margin: 0px;
text-align: center;
line-height: 60px;
color: antiquewhite;
border-bottom: 2px solid black;
padding-top:10px;
}
nav ul li{
  float:left;
  list-style:none;
  margin:20px;
  line-height:10px;
}
a {text-decoration:none;}
ul {text-align: center; display: inline-block;}
#content {
float: left;
width: calc(80%- 20px);
background-color:green;
min-height: 500px;
padding: 10px;
}