Bunu direk yapıştır dene bi bakalım;


body{
margin: 0px;
}

header{
width: 100%;
height: 100px;
background-color: #4286f4;
}

section{
width: 1200px;
height: auto;
background-color: #26d6d6;
top: 110px;
left: 0;
right: 0;
margin: auto;
}
section:after {content: ".";display: block;height: 0;clear: both;visibility: hidden;}

article{
width: 500px;
height: 500px;
float: left;
margin-left: 20px;
margin-top: 20px;
background-color: #07bc13;
}

footer{
width: 100%;
height: 100px;
background-color: #cece0c;
bottom:0;

}