1. Does the book cover how to write CSS and HTML code so that old browsers that do not yet support CSS3 and HTML5 would also render content gracefully?
2. Does book focus on rendering in mobile devices as well?
1. Each chapter has a browser compatibility section at the end which tries to answer those sort of questions, but it doesn't dive deeply into it as the focus is on the new features. You would need to do some further experimentation over and above what is shown in the book in several cases.
2. Not really, there is some discussion of it in chapter 8 (CSS3 layouts), but most of the examples are not really complicated enough that you would see too much difference on a mobile device.