The Blogs at Penn State generally produces accessible HTML....but that doesn't mean it couldn't use some improvement.
If you are as obsessed with CSS as I apparently am, you may be interested in reading how you can tweak the theme CSS templates to get a really nice display of H3 and H4 in blogs (these are for subsection headers of long posts or portfolio pages).
CSS or "Cascading Style Sheets" are recommended as a way of allowing the greatest number of formatting choices in Web design while writing better structured code. Below are some CSS tutorials which may be of assistance.