Edit in JSFiddle

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");

<div data-celda = "50%"></div>
<div data-celda = "25%"></div>

[data-celda]{
    height: 5em;
    background: green;
    margin-bottom: .5em;
}