It is important to note that HTML5 implies a transitional period between HTML 4.01 and the new standard. Furthermore, even HTML5-aware web browsers do not support the HTML5 specification perfectly. This means that you need to provide some sort of backward compatibility with all HTML5 features you use, providing that you do not want to alienate your users based on their choice of web browser.
This book stresses backward compatibility wherever possible. However, you should discourage your users from using outdated web browsers so that you can avoid these types of compatibility issues. You will learn how to best use an HTML5 tag and how to safely implement its HTML 4 fallback.