The previous version of HTML, HTML 4.01, came in 1999, and the internet has changed significantly since then.
HTML5 was designed to replace both HTML 4, XHTML, and the HTML DOM Level 2. It was specially designed to deliver rich content without the need for additional plugins.
The current version delivers everything from animation to graphics, music to movies, and can also be used to build complicated web applications. HTML5 is also cross-platform.
It is designed to work whether you are using a PC, or a Tablet, a Smartphone, or a Smart TV.