HTML5 has been receiving an extraordinary amount of attention, thanks in part to active support and promotion by Google, Apple, and Mozilla, among others. At every web conference, there’s an assortment of HTML5 related sessions, and they attract overflow crowds. Also the demands of HTML5 developers are increasing day by day.
Despite its high profile, however, the HTML5 specification isn’t even complete, much less officially blessed or broadly supported by browsers. It will probably be a decade before you can design sites that require a full range of HTML5 features with confidence that the vast majority of installed browsers will support it.
If you want to understand where the web is heading, and to be a part of its evolution, there’s no doubt that HTML5 is something you want to be on top of. The more of us who are using it in the early days and providing views and opinions, the better HTML5 will be when it matures.
But in terms of delivering practical value for sites you are building today, most of the HTML5 development process just isn’t yet relevant. There are a few exceptions mostly when you need to support multimedia on iOS devices.
This is not to say that there aren’t parts of immediate value; each feature needs to be considered separately. HTML5 is not a monolithic thing, but a diverse collection of features from which you can pick and choose.