In this paper, we present a system that supports the design of web graphical user interface by finding the optimal placement of interactive elements. The definition of optimal placement is context specific; it aims at maximizing measurable aspects of the user experience, and it is derived using expert knowledge embedded in the system, which is based on HCI principles, user studies, and data analytics. We use a novel modeling technique to represent the layout of web user interfaces. The model is unsupervisedly computed on information extracted by several image processing algorithms. The system identifies the website category, builds a layout model, compares it with the relevant optimal model, and recommends an alternative layout of interactive elements. To prove this concept, we present a study on e-commerce websites where placement of the checkout button has a significant impact on the online sale process conversion rate. The system identifies non-optimal placement, and it recommends an alternative position that is likely to improve the conversion rate. The results will be validated based on the Visa Checkout user experience. The system is implemented as an open-source software, and it currently supports the re-positioning of a single interactive element. In the paper, we discuss a further generalization of this approach to support a larger number of interactive objects in a wider spectrum of scenarios.