This research experimentally investigates whether Unity-in-Variety can account for the aesthetic appreciation of websites. In a first study we designed two sets of web pages, differing in layout style and content, to systematically and independently vary on both unity and variety via the design factors contrast and symmetry (for unity) and dissimilarity of elements and colourfulness (for variety). It was demonstrated that only the sets based on symmetry and colourfulness resulted in independent manipulations of unity and variety, respectively. These two sets of web pages were tested in Study 2 showing that, as predicted, both unity and variety independently and positively influence aesthetic appreciation. Following the principle of Unity-in-Variety, simultaneously maximizing unity and variety leads to an optimal balance where aesthetic appreciation is highest. Our research lends further support to the principle of Unity-in-Variety, extends it to the domain of HCI, and provides directions on how to purposefully design for an optimal balance between unity and variety.