Learning What’s Different with HTML5
HTML 4 is the last recommendation developed by the W3C alone. Most web pages right now are built in HTML 4 because it is widely supported by web browsers and editors.
XHTML was created by rewriting the HTML 4.01 specification as XML, which means that all tags must be closed, the XHTML tags must be written in all lowercase, all attributes must have quotation marks around them, and tags must be nested without overlapping.
HTML5 goes back to a less restrictive version of HTML. End tags are no longer required for all elements, you can write in upper- or lowercase, and attributes don’t need to have quotations around them all the time.
HTML5 also adds a lot of new elements, including a streamlined doctype (or DTD—the first line of your HTML document. It tells the browser that this document is an HTML5 one), sectioning elements, many new form features, and support for drag and drop and other features useful for creating web applications.