If you're going to require classnames anyway, it might be more flexible to use those to define the type of content to insert. Personally I'd prefer adding a simple bit of js to a page to define the fixie behaviour, something like :

  fixie.fill('.my_header_class','head');
  fixie.fill('#my_id','paragraphs');

etc, as otherwise I have to have a special layout with lots of 'fixie' everywhere and perhaps a special structure using certain tags in order to use the js, and can't define what sort of content goes where except by changing the type of html element which contains it.

I'd prefer to start with an html document which has already been made, and consider how to add fixie to it in the least obtrusive way - and how to remove it later when it was not required, without impacting the document structure or classes etc at all. So if I can add a bit of js which adds fixie in an unobtrusive way to any existing page, and then remove it by simply commenting out that js, that would be preferable to having to set up a special template just to use fixie, or having to consider fixie use from the start. Fixie should really IMHO adapt to the html it is used on, not force that html to be adapted to it.