HTML has been around for a couple of decades, but it only recently underwent significant syntax changes. The new HTML5 specification clears out some obsolete elements and adds new markup elements for specific (and common) tasks. New elements have stronger semantics that make it obvious what they are for—such as header, footer, menu, section, and more.
These new elements, however, live side by side with older and, semantically speaking, more generic elements such as DIV. The result is that sometimes you have two or more ways to achieve the same rendering—using direct HTML5 elements or a combination of more generic elements. If you plan to target HTML5 browsers, using new elements keeps your markup easier to read and understand—in a word, simpler.
The purpose of this book is to build Windows 8 applications, as opposed to classic websites, which makes the differences between older HTML and HTML5 unimportant: this book uses HTML5 all the way through. However, if your goal is to build a website for the general public, then integrating HTML5 in the markup of the pages is much more difficult. For web applications, you will need to deal with browser differences and ensure that the behavior is uniform across major browsers.