flex.css

*{
    box-sizing: border-box;
    border: solid red 1px;
}

html, body{
    margin: 0px;
    padding: 0px;
}

.flex-container{
    display: flex;
    flex-direction: column;
}

.center{
    display: flex;
    flex-direction: row;
    width: 100%;
    flex-wrap: wrap-reverse;
}

.left{
    flex-grow: 1;
    flex-basis: 0;
    min-width: 400px;
}

main{
    flex-grow: 3;
    flex-basis: 0;
    min-width: 400px;
}

.right{
    flex-grow: 1;
    flex-basis: 0;
    min-width: 400px;
}

footer{
    height: 100px;
}

header{
    height: 200px;
}