The way I understood it, HTML will validate with self closing elements but is not recommended.
further explanationSince the WHATWG blog is via WordPress...
I will say this, given that I have used WordPress, and for the most part find it very useful as a quick,simple blog framework.
The problem is that WP insistently force formats your posts in to XHTML markup (which it the current markup that WordPress uses last I checked).
I guess a quote that somewhat backs this up is on the

WHATWG Wiki:
http://wiki.whatwg.org/wiki/FAQ#Should_ ... _or_.3E.3FShould I close empty elements with /> or >?
Void elements in HTML (e.g. the br, img and input elements) do not require a trailing slash. e.g. Instead of writing <br />, you only need to write <br>. This is the same as in HTML4. However, due to the widespread attempts to use XHTML1, there are a significant number of pages using the trailing slash. Because of this, the trailing slash syntax has been permitted on void elements in HTML in order to ease migration from XHTML1 back to HTML.
It's much of the same issue we have with the forums. They are built in XHTML, we can edit parts of the STATIC code/markup but some of the deeper parts of the application will remain XHTML. Especially the parts that are dynamically created, like posts/comments.
I think the biggest issue here although being wrong in terms of markup, is that we are using third party frameworks that do not yet support true HTML and are still using XHTML.
Until we/WHATWG can find the time and resources to build an entire web based infrastructure from scratch, we can't really complain about frameworks formatting parts of our applications in less-recommended markup.
That being said, I would be first to sign up and help in any way possible if WHATWG were to start an official movement to unify all of the web based community resources in to true and proper HTML. In other words a new,complete professional website with all of the current recourses in one place.
The experience personally for me would be great as I work on my Comp Sci. degree and career goals.
But it is of course a relatively huge project with several applications needing to be created (Forums, Blogs, Online reading of Mailing list, Spec documentation, etc). Would take a team of developers/testers to ensure safe/secure applications.
/end
