section{
    background: #F3F3F3;
    padding: 30px 10px;
     
    display : flex;
    flex-direction : row;
    justify-content : space-between;
}
article{
    width : 70%;
    height : 100%;
    flex : auto;
    text-align: justify;
    padding: 10px;
    background: #FFF;
}
aside{
    padding: 10px;
    width: 30%;
    height : 100%;
    flex : auto;
    background: #FFF;
}
@media screen and (max-width: 1024px){
    section{
        flex-direction : column;
    }
}

Source = https://openclassrooms.com/forum/sujet/float-en-responsive