Although XHTML 2.0 isn't ready for general use yet, understanding how the changes will affect your writing and code will be key to a smooth transition. With full XForms integration, separation of content and function, and other features, XHTML 2.0 promises to revolutionize the Web when XHTML 2.0 arrives.
The previous versions of XHTML, XHTML 1.1 and 1.0, were made to be pretty much compatible with HTML 4.1. The problem with this was that it left a lot of presentational information in the Document Type Definition (DTD). Using the <b> tag says that you want to make a certain piece of text bold. However, the idea of XHTML is to separate content and layout.
Therefore, all presentational aspects of XHTML are completely gone in the upcoming 2.0. There is no "Transitional" or "Frameset" DTD -- both concepts are gone. (Although frames can still be used with XFrames, this will not be discussed in this article. The point is that, even with this, it will still use the same DTD as a normal site.) XHTML will include no presentation, requiring it all to be defined using CSS.
XHTML 2.0 will not be backwards compatible, and no browsers support it yet. However, more will in the future and this is the future of the web; this is where it is heading. Even now, an XHTML 2.0 page can be rendered on all browsers properly with some CSS tricks, so it is not obsolete because of this.
This article will cover the basics of XHTML 2.0 and why it is important. It is expected that the reader is familiar with XHTML 1.0/1.1. XHTML 2.0 has not been finished yet. (Currently, it is in draft form.) Keep in mind that there will be changes made between now and the finalization of XHTML 2.0.
HTML 
/ 28 
An Introduction to XHTML
