code: 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;
    }
docs: Determine is an element is visible somewhere within the document (page).
  
  @returns Boolean
  @private