HTML5 is the latest version of HTML, the Hypertext Markup Language that is used to define the content of web pages. It is closely associated with style sheets, which add style and layout control of the user interface, and JavaScript, which adds programmable interactivity to the web pages.

You can start learning about HTML5 here. THis is a short tutorial for complete beginners.

You can learn more about Style Sheets here again in a tutorial targeted a beginners to the subject.

You can then find an introduction to Javascript programming here. This tutorial also starts a the very beginning.