I didn't expect to learn so much, and that there are so many fundamental differences between web on desktop and mobile. The book has a really good introduction on the mobile world and the different browsers and devices out there. It dedicated a chapter for android which made sense since Android holds the bigger market share and because there are so many different browsers and devices for Android.
I didn't know there are different viewports to be honest, and there's a chapter that talks about them and the difference between them. Then it moves to talking about specific Javascript and CSS differences on Desktop and mobile in two different chapters. It also offered solutions and some snippets of code to different common problems.
I liked that it was fugue about some things that he knew will change from the time he wrote the book, like some compatibility issues for different browsers, but offered links to websites that will have the correct up to date data when relevant.
He ended the book with good advice on how to become a mobile web developer, if that's what you're hoping to be and you're not just reading this to be aware of main problems and how to fix them for your site. It also has a few pages about the future of the web, and he has some interesting ideas on mobile web VS native.