The motivation for our work is that even though user interface guidelines and style guides contain much useful knowledge, they are hard for user interface designers to use. We want to investigate ways of bringing the human factors knowledge closer to the design process, thus making it more accesible to designers. To this end, we present a knowledge-based tool, containing design knowledge drawn from general guideline documents and toolkit-specific style guides, capable of evaluating a user interface design produced in a UIMS. Our assessment shows that part of what the designers consider relevant design knowledge is related to the user's tasks and thus cannot be applied to the static design representation of the UIMS. The final section of the paper discusses ways of using this task-related knowledge.