State of the Art of Web Usability Guidelines Céline Mariage 1 , Jean Vanderdonckt 1 & Costin Pribeanu 2 1 Université catholique de Louvain (Belgium) 2 National Institute for Research and Development in Informatics (Romania) {mariage,vanderdonckt}@isys.ucl.ac.be, pribeanu@acm.org 1 Introduction Today exists an considerable body of knowledge dedicated to the usability of Human- Computer Interfaces (HCI) of computer-based systems. This knowledge is typically referred to as usability guideline, or guideline for short. Guidelines can be found in many different formats with contents varying both in quality and level of detail, ranging from ill-structured common sense statements to formalized rules ready for automatic guidelines checking. Guide- lines are particularly appropriate for consolidating usability knowledge (Stephanidis & Ak- oumianakis, 1999) existing for various aspects such as training workload and the universal accessibility. The importance of guidelines was first revealed during the eighties (Reed et al., 1999) when the use of the computer dramatically increased in the work place: more computer- based systems were used by more users, who were not necessarily expert, for more interactive tasks, possibly new or unusual. This situation progressively required more attention paid the usability. The advent of Web sites and Web-based applications raised the amount of sources containing guidelines for Web User Interfaces (UIs). Among others were published seminal books such as Nielsen (2000, 2002), style guides produced by individuals such as (Lynch, 1999) or by organizations such as (Sun,1999), standards such as ISO (1999), sets of design rules, and lists of principles. Using usability sources is not straightforward for developers and evaluators, primarily for some of the following reasons: 1. Usability remains a quality factor of user interfaces that is still handled with some uncer- tainty. Applying guidelines are a necessary condition, but not a sufficient one: the respect of guidelines certainly contributes to improve the usability of a web site, but a web site that is compliant with all possible guidelines may still be experienced as unusable by some end users. 2. Identifying in the jungle of guidelines which ones need to be addressed for a particular web site for a given target audience remains challenging. It is hard to select guidelines ap- propriate to a particular context of use since guidelines address many different issues: some guidelines are related to writing code (e.g., how to write pretty and syntactically valid HTML) while others encourage to support the variety of existing Web browsers to ensure accessibility. In this philosophy, the Web should be usable through any browser by