Obviously, it's about HTML in general and HTML5 in particular. It's also about how to think about, plan, design and develop for the Web.
I tried to write it so that it would be understandable by content creators and editors – People who are using the Web to publish rich content using services that allow some HTML markup, such as: Google Docs, Blogs, eBay and Wikipedia. But, there's also a chapter on building websites from scratch.
Mostly though, I've tried to provide guidance based on my 17 years of developing Web sites for individuals, small businesses and organizations.