In my thesis I present the client-side designing and developing process of an online banking system. I examine the stucture of web applications and their raison d'etre on the software market, furthermore I examine the modern web technologies and their support across the most popular web browsers. In the essay I introduce the main elements of the design, emphasizing on browser independence, multilingual support and the saving and recovering of states. In my thesis I also present the factors that influence the speed of the applications, and the technics for their optimization. I give detailed information about my security measures for making the system safe, and the most common attacks against web applications. Thereafter I scrutinize the web ergonomics and usability of the user interface, including readability and navigational structures. Then I discuss the implemented functions that were to enhance user experience. I conclude my essay by presenting two featured modules of the system.