I do not agree with the described concept of CSS id versus class. I think that page-mod-forum-view should be CSS *class* as it applies on all pages that display any forum main page. The CSS id should be use if the themer wants,for some reason,to have a special CSS for a particular instance. Therefore there should be id="page-mod-forum-view-42" class="page-mod-forum-view page-mod-forum page-mod" where 42 is cmid of the currently displayed module.
Similarly, page-course-view is definitely a class and it applies to all /course/view.php pages but page-course-view-5 is CSS id of one and only one course page (Using Moodle in case of moodle.org).
This would lead to a clear guideline on when to use ids (?id=xxx is passed to the script so we talk about one instance of the "page") and when to use classes (as class is always a set of all instances - pages in this case - of the same type).
I am pretty sure it is not late yet to rethink this. Better than live with wrong design for all 2.x ages... --David Mudrak 22:06, 31 March 2010 (UTC)