Testimonial section CSS inheritance and proper use of HTML5 elements
Everything looks clean. However, I found some things though:


font-size - Make the units consistent. You use px and em. I suggest you go em all the way.
background-size - Well, it exists for that reason, but it would be better if you actually sized the image in those dimensions rather than having them resized in CSS.
You "namespaced" everything by prefixing .testimonials. That's being a good developer. You're avoiding your styles from clobbering other elements. However, you forgot to do that for .author and p.