Accessibility and performance are often an afterthought when developing websites, unless you're a major e-commerce shop where every additional millisecond in page load time and every lost user result in less conversions.
I've been following the work of Filament Group for some time and was a big fan of their book "Designing with Progressive Enhancement" which I think every web developer should have read, so many of the topics discussed in this book weren't entirely new to me.
I think that many of the best practices presented here are essential for public-facing websites and sites that provide content rather than functionality. With the rise of web apps it's hard to say if all the principles of progressive enhancement can be applied to single-page application as well because, honestly speaking, that's kind of hard.
Things like feature detection and supporting older browsers and devices is an expensive trade-off if you want to move fast and ship often. It would have been interesting to hear the author's opinion on whether that's feasible or not, and how Filament treats web apps compared to sites that can work on a bare-bones HTML construct as well.
Then again, the book series focuses on conciseness and effective delivery of the most important bits of information. In this respect, Scott Jehl did a decent job and inspired me to be more diligent, systematic and thorough when it comes to developing and delivering responsible web experiences, even though we all know that can sometimes cause a lot of headaches :)