JSFiddle - React, Tailwind, and code Playground

HTML

<header>HEADER</header>
<main>
    <div class="left">Columna 1</div>
    <div class="right">Columna 2</div>
    <footer>Footer 2016</footer>
</main>

CSS

* {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}

html,
body {
    width: 100%;
    height: 100%;
    text-align: center;
    margin: 0;
}

header {
    border: 1px solid grey;
    width: 100%;
    height: 15%;
    margin-bottom: 10px;
}

main {
    border: 1px solid grey;
    width: 100%;
    height: calc(85% - 10px);
    padding: 10px;
    display: flex;
    flex-wrap: wrap;
}

.left {
    border: 1px solid grey;
    width: calc(70% - 10px);
    height: calc(85% - 10px);
    margin: 0 10px 10px 0;
}

.right {
    border: 1px solid grey;
    width: calc(30% - 10px);
    height: calc(85% - 10px);
    margin: 0 0 10px 10px;
}

footer {
    border: 1px solid grey;
    width: 100%;
    height: 15%;
}