You should understand by now that the multi-device web is unknowably vast and varied, that the range of browsers running on those devices is immense, and that even within those browsers there is a variety of versions and implementations (I hope you understand that, as most of the introduction was dedicated to trying to get that point across). That being the case, some of the features in this book may well not be implemented or may be implemented slightly differently.
Rather than covering the levels of implementation in the text, I treat every new feature as if it’s fully implemented and make a note of real-world implementation oddities and curiosities in Appendix M.
Also, cutting-edge standard proposals are subject to change, even when experimental implementations have already shipped in some browsers (the Grid Layout module featured in Chapter 4 was updated while I wrote this book), so by the time you read this, some of the syntax in the book may already be out-of-date. This is an unfortunate risk when working with evolving standards in dead tree publishing, but I try to mitigate it by noting features that may be at risk of changing and by keeping a list of errata and updates on the companion website, http://modernwebbook.com/.
You’ll probably want to follow a few online resources to learn about levels of implementation, although most tend to be focused on desktop and mobile browsers. Can I Use… shows levels of support for a wide range of technologies in recent past, current, and future versions of popular browsers, whereas HTML5 Please shows how safe it is in general to use cutting-edge features, mostly CSS3 and JavaScript (which is why the name annoys me a little).
The HTML5 Test is a site that tells you how many features from the HTML5 spec your browser supports, but also very usefully keeps records of implementation levels across many different browsers and devices, including TV and games console browsers, and also allows you to do side-by-side comparisons of up to three different browsers. The site is limited to only HTML5 support, however.