In earlier types of client-server computing, each application had its own client program which served as its user interface and had to be separately installed on each user's personal computer. An upgrade to the server part of the application would typically require an upgrade to the clients installed on each user workstation. In contrast, web applications use web documents written in a standard format such as (X)HTML, which are supported by a variety of web browsers. Generally, each individual web page is delivered to the client as a static document, but sequence of pages can provide an interactive experience. During a session, the web browser interprets and displays the pages and acts as the universal client for any web application. The ubiquity of web browsers and the convenience of using a web browser as a client, sometimes called a thin client, without the need to install software, make web applications desirable.