What is the meaning of the word 'navitis' in the second edition of 'Introducing html5'? [closed]

I would take this almost certainly to mean "overuse of the <nav> tag". The suffix -itis comes from the Greek language and is taken to mean "inflammation of" in modern English. However, in an idiomatic sense, adding -itis to a word means that the author believes that the action is being done too often. In this case, the author is referring to the overuse of the new <nav> tag, like it is a disease.

The author defines the term in the very sentence you quote:

I was previously guilty of navitis—the urge to surround any links to other parts of a site as <nav>.

The author is using a standard form for definition of an unfamiliar or coined word in context.

I guess that navitis is a word the author invented to mean "too many tags". He probably defines exactly what he means in one of the early chapters. E.g.:

Just look at all those sections. It's so ugly! This is what I call navitis.

-itis is an informal / humorous suffix which means "disease / illness". (In formal usage, it's a medical suffix that means "inflammation".) So he's describing a "disease" of website design.