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

#container {
  width: 1100px;
  margin: 0px auto;
  border: 4px solid gray;
}
/*SEGUNDO PASO*: Configurar Encabezado */
header {
  background-color: red;
  height: 100px;
  width: 100%;
  margin: 0px;
  text-align: center;
  line-height: 100px;
  color: antiquewhite;
  border-bottom: 2px solid black;
}
/*Tercer Paso: Crear Menu*/
nav ul li {
  float: left;
  list-style: none;
  margin: 20px;
  line-height: 10px;
}
/*Quitar Subrayado */
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;
  /*Usando la función calc*/
  width: calc(20% - 20px);
  background-color: coral;
  min-height: 500px;
  padding: 10px;
}

/*Quinto Paso: Modicar el Footer */
footer{
  background-color: black;
  color:white;
  text-align: center;
  height: 30px;
  line-height: 30px;
}
.clearfix{
  clear:both;
}
* {
  margin: 0px;
  padding: 0px;
}