For the joint project ‘optes’, a web-based pre-course in mathematics was developed, addressing knowledge gaps in basic mathematics. The course concept comprised a diagnostic pre-test, learning modules, animations, exercises, and a final self-assessment. STACK questions were used in training sessions, each question providing feedback and detailed sample solutions. While the flexibility of this question type was found highly useful for training purposes, there were some concerns if students in a self-study environment would have difficulties with the input syntax AsciiMath. An evaluation study carried out by one of the optes partner universities focused on this issue. The results suggest that STACK questions are highly suitable for self-study learning environments and can be combined with other question types. Based on these experiences some suggestions for the design of web-based courses are made.