Columns of text are narrower than a book, to make reading on the screen easier. Navigation system is easy to understand and use. Navigation is consistent throughout the site. Large sites have an index or map. Link colours coordinate with page colours.
Links are underlined, so they are clear to a visitor.
Buttons are not too big. Every graphic has an alt label. Every graphic link has a text alternative. Animated graphics turn off by themselves, or better still are not used. Pages download quickly.