_isElementVisible = function(el) {
    if (!el) {
      return false;
    }
    var styles = _getComputedStyle(el, null);
    if (!styles) {
      return false;
    }
    var hasCssHeight = _parseFloat(styles.height) > 0;
    var hasCssWidth = _parseFloat(styles.width) > 0;
    var hasCssTop = _parseFloat(styles.top) >= 0;
    var hasCssLeft = _parseFloat(styles.left) >= 0;
    var cssKnows = hasCssHeight && hasCssWidth && hasCssTop && hasCssLeft;
    var rect = cssKnows ? null : _getElementPosition(el);
    var isVisible = styles.display !== "none" && styles.visibility !== "collapse" && (cssKnows || !!rect && (hasCssHeight || rect.height > 0) && (hasCssWidth || rect.width > 0) && (hasCssTop || rect.top >= 0) && (hasCssLeft || rect.left >= 0));
    return isVisible;
  }
Determine is an element is visible somewhere within the document (page).

@returns Boolean
@private