Today, many factories use robots for intensive and repetitive work. Programming these robots is typically costly and time-consuming, and requires specialized expertise, hardware or non-expert employees interacting with expensive robots. In this paper, a new approach for teaching assembly tasks to robots is proposed. The user demonstrates the assembly task using a web browser in 3D and then simulates the robotic execution to promptly assess the outcome of the teaching process. A within-subjects user testing study compared the proposed simulation-based approach against a state-of-the-art computer vision-based approach. Participants were asked to teach a real-world LCD TV assembly task to a robot using both approaches. Results showed that users of the proposed simulation-based system had significantly lower time on task values and were significantly more likely to prefer it compared to the computer vision-based system. No significant difference was observed in task success rates and SUS scores.