I am reminded of the quote “The nice thing about standards is that there are so many of them to choose from.” attributed to “Andrew S. Tanenbaum”, whilst looking into the structure of web pages.
The Firefox browser add in ‘FireBug’ provides a nice tool under ‘Tools- Validate HTML’ which enables the page one is viewing to be checked by the W3C Markup Validation Service. By default it uses the standard defined at the start of most web pages, but the number and variety of possible standards is most interesting. The output is a nice listing of ‘errors' and ‘warnings’ upon the page structure and its elements. [Note that it excludes the ‘new’ standards mentioned below, although it does have ‘HTML5 Experimental’]
Whilst one can then proceed to correct and eliminate the errors and warnings it does raise the question as to what standard should one be writing web pages to: HTML5, XHTML 1.0 Transitional, XHTHL 1.0 Standard, XHTML 1.0 Framework, HTML 4.01 Strict etc. The list goes on to provide about 15 possible standards.