"Also, avoid using multiple fonts from the same font family. While the fonts look similar, the minor differences can throw your users off and make your site look cluttered."
I gotta disagree with this paragraph. I more often than not see websites with one font family. Combining bold (titles) with regular (body text). This looks perfectly fine. The above is true when using two font families that look too much like eachoother. In that case, it will 'throw your users off" and make the site look somewhat weird.