The Three Levels of HTML5 Usage
The HTML5 spec is written (and still being edited) with backwards compatibility in mind. There are some new features in HTML5 that already work in every A-grade browser and can thus be used today, without requiring any weird hacks. Level 1 HTML5 is limited to these features.
Redefined HTML4 elements
Some HTML4 elements were changed in HTML5. Because these elements already existed before HTML5 and have been around for a while, all browsers already support them. You really don’t have to do anything to benefit from their new meaning and added possibilities in HTML5.