Assessing the cognitive engagement of a user while seeking and consuming information over the World Wide Web is a key challenge for studying the quality of interactions. Indicators of cognitive engagement are useful for enhancing usability of interfaces, designing adaptable systems but also for analysing user behaviour and performance. For this purpose, we aim to adopt the multifaceted concept of Human Mental Workload, mainly applied in psychology and cognitive sciences, to study individual performance and user engagement in the context of Web. We aim to design a framework in which mental workload can be measured, analysed and explained. This will lead to analysis of individual and mass behaviour, guidelines and recommendation for interaction design, usability of front-end web applications and proposal of adaptive systems.