Sorry I didn't notice earlier that you had questions I could've answered, Steve. I just saw the playing around in the TOC and didn't dig into it.

There is a filter on HTML in posts. It's applied at the server when you submit the post, and so its effects don't show up in preview. Things that might seem to be allowed by preview can get still get nuked by the filter when it processes your post.

The filter is a based on a long list of allowable HTML tags. I checked the list just now, and to my surprise I found that I've blocked the DIV tag. According to some comments accompanying that block of code, I had a mental debate about whether to allow it, and finally went on the side of caution. Too much caution, I now think in retrospect. The possible abuses of DIV are few and the benefits many.

I goofed in hardwiring that list into the software, so at the moment I can't easily restore DIV to posts. I'll make a note to review the filter next time I work on the server software.

In the meantime, SPAN is allowed, and P for most practical purposes can fill in for DIV. P is just a particular type of block element, and DIV represents the generic class of block elements.

For guidance about what you might want to do with it, when I worried about whether to block DIV I was thinking of ways that it could be used to hijack the page and take over its appearance. A DIV could be positioned to obscure some standard part of the page and overlay it with something else. I consider the elements of page design out-of-bounds and to be protected from modification by scripts or HTML in posts, so please don't use it that way. Using DIVs to add blocks of color or specify different fonts or other decorative purposes, hey, knock yourself out.