Keeping content looking and behaving like the HTML elements used to describe it is great for reinforcing external consistency. Externally consistent content is great for ensuring people can, and will use your website or webapp.

Familiar interfaces borrow from well-established patterns. These should be used consistently within the interface to reinforce their meaning and purpose. This should be applied to functionality, behavior, editorial, and presentation. You should say the same things in the same way and users should be able to do the same things in the same way.