HTML's newest member of the family is on its way. HTML 5 brings to the table a unifying system and language to the web design world.

Evolution of Language
Since its introduction to the internet in the early 1990's, HTML has constantly been a changing, evolving language. Much has changed in the past 19 or so years. The way people use the internet has changed, and therefore, so must the way code is written. So far, there have been four official versions of the HTML language. In 1997, HTML 4 became a World Wide Web Consortium (W3C) recommendation. Since then, HTML has almost become the "official" language of web design. XHTML is a language that specified HTML further to conform to the strict guidelines of XML. These languages have been the basis for many web design projects and integration of plug-ins from third-party companies in the years that they have been around. Work on HTML 5 began in late 2003, to prove that the expansion of HTML 4 was possible. Since then, some early drafts have been released, Apple, Mozilla, and Opera have announced their intent to continue working on the project, and in 2006, W3C created a working group to work with WHATWG to develop the specifications of HTML 5.

What HTML 5 Offers
This specification evolves HTML and its related APIs to ease the authoring of Web-based applications. Additions include context menus, a direct-mode graphics canvas, a full duplex client-server communication channel, more semantics, audio and video, various features for offline Web applications, sandboxed iframes, and scoped styling. Heavy emphasis is placed on keeping the language backwards compatible with existing legacy user agents and on keeping user agents backwards compatible with existing legacy documents.( WHATWG HTML 5 Abstract)

Pros

1. Backwards Compatible
   HTML 5 allows for reading, viewing, and editing of previous versions of HTML and XHTML scripts. This makes it easier for websites that have already been designed using an older version of HTML, because the programmer can just change a few things as opposed to having to re-write the entire site.
2. Defines Both HTML and XHTML
   As HTML 5 incorporates all of HTML and XHTML, there will be no need for two languages now. This unification of the languages allows for programmers to only need to be familiar with one language while still having all the same variety of options as before.
3. The End of Plug-ins
   HTML 5 incorporates programs similar to Flash, Java, and Silverlight, thus eliminating single-corporation domination. This form of open-source plug-ins will allow for a large room for growth in this field.
4. Compatible with Many Browsers
   Safari, Opera, Chrome, and Firefox have already begun incorporating the finished parts of HTML 5 into their browsers. Microsoft has voiced its support of the HTML 5 project, but has not begun any integration. With such a broad acceptance among web-browsers, HTML 5 will appeal to programmers because of the ease of access between browsers.
5. Potential
   It is said that HTML 5 has the potential to eliminate, or at least diminish the roles of OS, and to reduce the chances of a monopoly on the internet and related programs, because of its integrated programs. This potential will help push the imagination of programmers world-wide to create new and better programs.

Cons

1. New Language
   There have been a few changes to the HTML language to accommodate these changes, and to make it more user-friendly. This is really supposed to make the language more user-friendly and logical, which would really make this more of a pro. There really should be no problem with the slight changes, but they may prove a slight inconvenience for programmers that already know the HTML and XHTML languages and have to relearn a few things..
2. Time
   Though certain aspects of HTML 5 are complete already and functioning on certain websites, there are many, many more features that have yet to be programmed, or even thought of. HTML 5 is still under development, and will likely still be for a good amount of time. Don't expect HTML 5 to become a W3C recommendation any time soon, it's still got a few years.

Conclusion
HTML 5, though still in development, has the potential to open programming to a new audience. With its user-friendly language, open-source built-in "plug-ins", and combination of previous HTML and XHTML languages, HTML 5 will be the best mark-up language to-date. The compatibility between browsers increases the effectiveness for programmers, and saves precious time.

Further Research
5 Amazing HTML 5 Features to Look Forward To
HTML 5: Could it kill Flash and Silverlight?
HTML5 Could Be the OS Killer
W3C Official HTML 5 Website
Differences Between HTML 5 and HTML 4