HTML was developed as a standardised approach to information mark-up and new versions have certainly been developed over the years to increase complexity, readability and to take account of the increasing use of graphics and presentation techniques. However if we compare XHTML, which the World Wide Web Consortium has focussed its effort on since 1999, to HTML the most important changes have been in terms of structure, rather than in terms of content. Many of the familiar tags in HTML remain in XHTML, although some have been depreciated and some added, an HTML developer of the 1990s (such as myself) would not notice a great difference with the new standards. Indeed, if a developer of other mark-up and programming languages, the new structures in XHTML may well have been ones that they were following in any case under HTML as they are required by other languages; proper nesting, tags closed and lowercase elements.
An important aspect in the importance of HTML over newer versions is also browser support. Many browsers still support HTML and will continue to do so as long as there are a significant number of sites out there that still use HTML and given that there will be no requirement in the foreseeable future for a web site owner to migrate to XHTML, it is likely that backwards compatibility will remain for some time. As newer web developers and designers enter the industry and those already working update their knowledge, there will be a gradual movement to XHTML as it does offer advantages over HTML, however, in my opinion, as the content on the web is so vast it will be many years, if not decades before HTML is a tiny minority. The main driver of this will be when a widely used browser moves over to dedicated XHTML support only and publishers who are still interested in their sites being browsed then make the changes.
In addition to XHTML, it must also be remembered that the web uses various other technologies to generate web pages and database driven web sites that use languages such as PHP or ASP (programming languages capable of performing more than simple text mark-up) also generate standardised HTML or XHTML code for the browser to interpret. These systems tend to be updated to the latest standards as code generators tend not to replicate the page code so it is simpler to modify thetag to <XHTML….. > in one place (hopefully) and validate the code, which will then affect all generated pages. This process is much more ungainly on sites with static pages.
In conclusion, I believe that HTML has outlived its usefulness for developers and should not be used however in terms of the user experience and changing what it out there on the Internet, it will continue for a long time.
Deitel, P. & H. (2010) Internet & World Wide Web: How To Program (4th Edition). Pearson Prentice Hall.