The visual beauty of an interface, or indeed everything else, is dictated by the harmony of its parts. Should every element of the composition work in concert to support one another, with nothing sticking out of place or asking for undue attention, with no treatment of style or spacing that does not follow the motif; should they merge together into a harmonious whole with a clear theme and overall effect, with all the elements on the page or screen coming together into a single unit of the interface as one whole; should these qualities be present we will have a beautiful design.
For a composition to be beautiful it must be harmonious and balanced, with any imbalances created specifically for desired effect. Unintended imbalances are almost always out of place unless they are balanced out by other imbalances – e.g. rough, hand-made produce may have uneven surfaces, but because the whole thing is uneven the roughness of the surface ceases to matter, and indeed, becomes a sort of texture in itself, rather than it being a visible flaw. Nature is beautiful because it is harmonious, and even though there are theories on a landscape being attractive to us due to the presence of certain things (river, grass, trees, shelter) for evolutionary reasons, the very composition of those parts gives rise to harmony (i.e. landscape paintings don’t usually involve a flat line of grass, but a composition of various elements, and as for water paintings, then the artist works with the waves which form their own pattern of increasing size).
Why does harmony seem beautiful to us? Perhaps our brain enjoys establishing a pattern of what we are seeing, and enjoys it all the more seeing it come true – seeing its deductive power triumph at grasping the invisible framework of the thing in question. If our brain fails to understand the underlying design, or if elements are found to be outside of a projected framework, then we experience a negative feeling, either a feeling of being unable to understand a work, or a feeling of being able to understand the design but seeing flaws in the elements that do not conform to it. In this way, a harmonious design is easy to grasp as a whole. As you experience it you find no element that is out of place, nothing that sticks out from the framework you understand the design to follow. It gives us a feeling of rational order, and a confirmation of our own cognitive abilities, a positive feeling of being able to make sense of the world. Discordant designs, however, create confusion and a feeling of unfulfillment – we can never grasp the design in full, either because the whole thing is confusing, or because certain elements break the perceived concept.
The complexity of material dictates the difficulty of achieving harmony. The more complex the content the designer has to work with, the harder it is for them to give the work a perfect sense of order. Complex material that has been brought under the control of a harmonious framework has the potential to give us more pleasure because we can feel not only the elegance of the final work, but also the great amounts of effort that went into making it. Just as hand-made goods exhibit not only their design but the physical labor that went into their making, work in which visual complexity has been given order through a sense of balance and harmony exhibits the intellectual labor of its creators.
The beauty of an interface, then, is achieved through simplified complexity – simplified in the sense of being arranged around a framework that helps the observer grasp its motif and meaning rather than merely reduced to a few parts. Organization (through style, spacing, color, etc.) can reduce the work into smaller units without reducing the number of units present, and the process of this intellectual reduction, the process of bringing a cloud of elements under control by giving them definite shape and contours, giving them groups and meaning, is the process of bringing order to complexity – the process of interface design.