The skinning document (see the download section on this site) has a list of all the attributes you can assign a class to. I also tells how the way the menu has been built in terms of tables and table cells.
Some attributes have a default class assigment (e.g. MainMenu_MenuItem), some do not.
Whether you need to use ".rootmenuitem" or ".rootmenuitem TD" depends on what you want to achieve. But my guess is that (some) people who are occasional skinners are wading their way through the many ways that lead to Rome. Most will stop analyzing when they found a working solution.
Using ".rootmenuitem TD" instead of ".rootmenuitem" causes a higher specificity in the CSS cascade. So if you have competing styles in the various .css files (default, skin, portal), it wins. Which may exactly be what someone wants.
I usually define the attributes I need myself, e.g.
<dnn:MENU runat="server" id="dnnMENU" display="horizontal" rootonly="false" separator="|" separatorcssclass="user" menucontainercssclass="CW0menucontainer" menubarcssclass="CW0menubar" menuitemcssclass="CW0menuitem" menuitemselcssclass="CW0menuitemsel" rootmenuitemcssclass="CW0rootmenuitem" rootmenuitemactivecssclass="CW0rootmenuitemactive" rootmenuitemselectedcssclass="CW0rootmenuitemsel" rootmenuitembreadcrumbcssclass="CW0rootmenuitembreadcrumb" submenucssclass="CW0submenu" submenuitemselectedcssclass="CW0submenuitemsel" submenuitemactivecssclass="CW0submenuitemactive" submenuitembreadcrumbcssclass="CW0submenuitembreadcrumb" />
In this way I know default styles will not interfere. Default classes I don't need, I give the setting: { display: none; }
Now the order in which you define the styles in the css also plays an important role, so I first define the CW0menuitem class. This class gets applied on the submenu items. Then I define the CW0rootmenuitem class, it defines the settings for the root menu (and overides the CW0menuitem).
Usually I don't need to use, things like: CW0menuitem TD.
Your current Item class (I think you mean: CW0rootmenuitemactive and/or CW0rootmenuitembreadcrumb) need be defined below the CW0rootmenuitem for them to have an effect.
Peter