Elements to consider: visual guidance, size of display elements, window hierarchy, brightness and saturation, appropriate typeface,...

For example, during a usability-testing study, a participant encountered many issues while shopping on the FitBit site. The issues ranged from minor annoyances in the interaction design to serious flaws in the navigation. She was able to complete her task, but with difficulty.

However, in a post-task questionnaire, she rated the site’s ease of use... See more