From the website:

In its ruggedness and lack of concern to look comfortable or easy, Brutalism can be seen as a reaction by a younger generation to the lightness, optimism, and frivolity of today’s web design.

What is “butalism” (llama2):

Brutalism is an architectural style that emerged in the 1950s and gained popularity in the 1960s and 1970s. It is characterized by the use of raw, unfinished concrete and other materials to create structures that are often massive, monolithic, and imposing. Brutalist buildings often feature bold, geometric forms and a lack of ornamentation, giving them a cold, imposing appearance. The style was associated with modernism and was seen as a reaction against the ornate and decorative styles that were popular in the early 20th century.