In web usability studies, a ‘contextual fidelity’ model identifies four factors that significantly affects testing accuracy: web aesthetics; testing environment and circumstances; participants’ personality traits and user states; and the nature of test task. In most works, these factors are considered independent; however, this has never been properly tested. To partly cover this gap, we experimentally test the one-directional effects of web aesthetics and task nature upon user states. Of dysfunctional user states described in literature, we have chosen monotony and anxiety, both equally damaging for user experience but differing by cognitive strategy. By combining high/low-quality web aesthetics and tasks directed to monotony/anxiety, we show that these two factors condition each other in users’ action. To detect monotony and anxiety, we measured cognitive efficiency and emotional stress. 48 student assessors divided into 4 groups were involved in the experiment. Our results shown that, counter-intuitively, the impact of website aesthetics on user states differs in cognitive performance and not in emotional stress. In the case of anxiety, high quality of design provokes hyper-lability of attention, does not contribute to its concentration, and reduces productivity. In the case of monotony, the aesthetic layout dramatically reduces the rate of fatigue and increases productivity. Our results prove that web aesthetics has a non-negligible impact upon more rational decision-making of users, which was overlooked in previous studies.