Wie rufe ich HTML-Elementstilwerte in JavaScript browserübergreifend ab?

<style>
#box {
  width: 100px;
}
</style>

<div>

document.defaultView.getComputedStyle(element, null).getPropertyValue(propertyName);

element.currentStyle[propertyName];

function getStyle(element, property) {
  var value, defaultView = (element.ownerDocument || document).defaultView;
  // Sanitize property name for different browsers
  property = property.replace(/([A-Z])/g, "-").toLowerCase();

  if (defaultView && defaultView.getComputedStyle) {
    return defaultView.getComputedStyle(element, null).getPropertyValue(property);
  } else if (element.currentStyle) {
    // Convert camelCase property name to hyphenated
    property = property.replace(/\-(\w)/g, function(str, letter) {
      return letter.toUpperCase();
    });

    value = element.currentStyle[property];
    if (/^\d+(em|pt|%|ex)?$/i.test(value)) {
      return (function(value) {
        var oldLeft = element.style.left, oldRsLeft = element.runtimeStyle.left;
        element.runtimeStyle.left = element.currentStyle.left;
        element.style.left = value || 0;
        value = element.style.pixelLeft + "px";
        element.style.left = oldLeft;
        element.runtimeStyle.left = oldRsLeft;
        return value;
      })(value);
    }
    return value;
  }
}