Layout and Composition
Formal balance, also known as symmetrical balance happens when the elements of a composition are the same on both sides of the central axis line.
Such symmetry, (horizontal symmetry) is not practical for all designs, but it can be used on website layouts when you would like to center content or balance it using columns.
Radial and bilateral symmetry are less common in web design, simply due to the constraints of the medium.
The way in which different elements in the composition interact and relate to one another is called unity. Unified layouts work better as a whole rather than in separate pieces or parts.
The proximity of objects to one another is also important. Placing objects close together makes them look like a single unified entity. Placing them further apart makes them individuals.
Repetition can be used to great effect to create a sense of unity across the page. Think of the way a shoal of fish appear in the ocean as a unified entity.