Also note that most links in this article will direct to w3.org or w3schools.com. Those are the two leading websites for any web based information you can find. Some other sites can provide more detail and are sometimes a bit clearer, but those two sites should be your first stop in the world of web development.
The HTML5 Umbrella Myth
Official Start Date of the New Web Standards?
While not yet accepted in final draft, HTML5 and CSS3 are supported to a good extend in most modern web browsers. The biggest exception to this is Microsoft’s Internet Explorer. They are usually two years behind in accepting most W3 standards. And even when the newest standards are accepted, there’s no law stating that browsers must implement every aspect. Until then, you can always start building web solutions for browsers that already support the greatly needed modifications. Here’s a rather comprehensive list.