body {
  background-color: lightgray;
  font-family: Arial, Helvetica, sans-serif;
}

#container {
  width: 1100px;
  margin: 0px auto;
  border: 4px solid gray;
}
* {
  margin: 0px;
  padding: 0px;
}
header{
  background-color: red;
  height: 100px;
  width:100%;
  margin:0px;
  text-align: center;
  line-height: 100px;
  color: antiquewhite;
  border-bottom: 2px solid black;
  }
  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;
}
.article{
  color:white;
  margin-top:15px;
  margin-bottom:15px;
  border-bottom:1px solid white;
  padding-bottom: 20px;
  }
  aside{
    float: left;
    width: calc(20% - 20px);
    background-color:coral;
    min-height: 500px;
    padding: 10px;
    }
footer{
  background-color: black;
  color: white;
  text-align: center;
  height: 30px;
  line-height: 30px;
  }
   .clearfix{
     clear:both
     }