Genial Alexis88 !!
Aunque parece que no me ha salido
Código HTML:
<!DOCTYPE html>
<html lang="es">
<head>
<meta charset="UTF-8">
<title>Document</title>
<script>
var celdas = document.querySelectorAll("[data-celda]"),
total = celdas.length;
for (var i = 0; i < total; i++)
celdas[i].style.width = celdas[i].getAttribute("[data-celda]");
</script>
<style>
[data-fila] {display:table; margin:0 auto; width:100%;}
[data-borde] {border-collapse:collapse;}
[data-borde]>[data-celda] {border:.2em solid black;}
[data-celda] {display:table-cell; vertical-align:top; padding:0 10px;}
[data-celda]{background: green; height: 5em; margin-bottom: .5em;}
</style>
</head>
<body>
<div data-fila="960px" data-borde>
<div data-celda="30%"></div>
<div data-celda="70%"></div>
</div>
</body>
</html>