The aim of this paper is twofold: firstly, to find the set of ontologies (i.e., the set of concepts and skills) presupposed by users when interpreting the meaning of web interface signs (i.e., the smallest elements of web user interfaces), and secondly, to investigate users’ difficulties in interpreting the meanings of interface signs belonging to different kinds of ontologies. In order to achieve these aims an empirical user study was conducted with 26 test participants. The study data was gathered by semi-structured interviews and questionnaires. Following an empirical research approach, descriptive statistics and qualitative data analysis were used to analyze the data. The study results provide a total of twelve ontologies and reveal the users’ difficulties in interpreting the meanings of interface signs belonging to different kinds of ontologies.