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; }