The "old" HTML/XHTML standards have a DTD (Document Type Definition) defined for them:
HTML 4.01 http://www.w3.org/TR/html401/sgml/dtd.html
XHTML 1.0 http://www.w3.org/TR/xhtml1/dtds.html#a_dtd_XHTML-1.0-Strict
This DTDs specify the rules for nesting elements - "which types of elements may appear in which types of elements". I made a diagram for XHTML 1.0 here (sorry, I no longer have that resource)
I would like to update that diagram with a new version which also includes the new HTML5 elements. However, there doesn't seem to be a HTML5 DTD. It seems that the nesting rules are defined by the various content models that are defined in HTML5.
So there is no DTD, correct?
Follow-up question: Is there a reason why there is no DTD in HTML5? The DTD is such a nice method of defining the nesting rules for all the different types of elements. Why wouldn't they include such a thing?
Update: I found this: http://www.w3.org/TR/html5/dom.html#kinds-of-content I guess, this is the closest to having a DTD.
Update: The Visual Studio Team made a XML Schema for XHTML5. I guess that answers my question: http://blogs.msdn.com/b/webdevtools/archive/2009/11/18/html-5-intellisense-and-validation-schema-for-visual-studio-2008-and-visual-web-developer.aspx
There is no HTML5 DTD. The HTML5 RC explicitly says this when discussing XHTML serialization, and this clearly applies to HTML serialization as well.
DTDs have been regarded by the designers of HTML5 as too limited in expressive power, and HTML5 validators (basically the HTML5 mode of http://validator.nu and its copy at http://validator.w3.org) use schemas and ad hoc checks, not DTD-based validation.
Moreover, HTML5 has been designed so that writing a DTD for it is impossible. For example, there is no SGML way to capture the HTML5 rule that any attribute name that starts with “data-” and complies with certain general rules is valid. In SGML, attributes need to be listed individually, so a DTD would need to be infinite.
It is possible to design DTDs that correspond to HTML5 with some omissions and perhaps with some extra rules imposed, but they won’t really be HTML5 DTDs. My experiment with the idea is not very encouraging: too many limitations, too tricky, and the DTD would need to be so permissive that many syntax errors would go uncaught.