This is great input, the named styles are my absolute favourite thing about Word. One of my other peeves with almost all WYSIWYG editors is that they default to line-by-line editing, and generally avoid exposing paragraphs as a first-class construct. In both Word and HTML, treating content as paragraphs makes things more predictable, stable (in terms of layout), and easier to understand IMO.
Aloha Editor uses paragraphs as first class construct. The https://github.com/alohaeditor/Aloha-Plugin-MetaView plugin gives you a nice outline view of the used dom elements to understand the semantics. Using tags for formattings like b and i seems a good choice to me as it is the suggested formatting tag for visual bold or italic presentation in HTML5. Other formattings like (indent, etc) could be implemented as css classes, but this could cause problems when copy/paste content from on site to another as the css classes may not expose the same way. This could lead to unwanted behavior because the normal users will understand the reason.
