Designing accessible web sites and more generally Internet-connected devices remains a challenging task nowadays. A number of guidelines (such as the WCAG2) are now widely available and recognised. To better cope with the quickly evolving technological landscape, these guidelines are also being formulated in technology-neutral terms. However this is still leaving the user dimension largely implicit, which makes it difficult to understand exactly which kind of user a given website is hindering.This paper describes how to capture and use rational links between guidelines and user capabilities/impairments by combining a set of complementary models (user, task, user interface, guidelines). The process of building those accessibility rationales relies upon available user and guidelines ontologies and also on obstacle identification and resolution techniques borrowed from the requirements engineering domain. This resulting enriched guidance enables a number of interesting new scenarios to better help web developers, analyse guidelines or make comparisons between guidelines.