Conflating the syntax of the setStyle and setStyles methods of node is a simple mistake that is difficult to track down and easy to make.

Y.one(document.body).setStyles("opacity", 0); // fails silently
Y.one(document.body).setStyle({ opacity: 0 }); // fails silently

The solution is either to unify the method:

Y.Node.prototype._oldSetStyle = Y.Node.prototype.setStyle;
Y.Node.prototype.setStyle = function (key, value) {
if (Y.Lang.isString(key)) { this._oldSetStyle(key, value); }
else { this.setStyles(key); }
};

Or to throw exceptions for incorrect usage of the methods.