In this paper we present ESK, an authoring tool for the automatic customisation of the front-end of web applications as a result of changes that users perform in dynamically generated HTM pages. Our authoring tool uses domain knowledge and presentation knowledge stored in PEGASUS, an automatic web page generation system for rendering ontology-driven knowledge. ESK automatically detects the differences between original and modified web pages and uses heuristics to infer additional knowledge for modelling the context of each change. ESK uses an explicit user model to identify which user can perform each kind of change to the web presentation.