Wikipedia is the one that immediately springs to mind as having a visually very basic design that does not put people off. Despite this, it actually has a lengthy Manual of Style (MoS) and indeed Wikipedia is among the ten most visited websites worldwide.
The MoS presents Wikipedia's house style, to help editors produce
articles with consistent, clear, and precise language, layout, and
formatting. The goal is to make the encyclopedia easier and more
intuitive to use. Consistency in language, style, and formatting
promotes clarity and cohesion; this is especially important within an
As soon as you get past the first page, you never have to relearn how to interpret the content on another page
(Yes there's graphics - sometimes - if the article provides it, but that's not part of the design - that's part of the content)