Wordpress - wp_nav_menu remove class and id from li

You can change the id and class of each item with the nav_menu_item_id and nav_menu_css_class filter hooks like so:

add_filter('nav_menu_item_id', 'clear_nav_menu_item_id', 10, 3);
function clear_nav_menu_item_id($id, $item, $args) {
    return "";
}

add_filter('nav_menu_css_class', 'clear_nav_menu_item_class', 10, 3);
function clear_nav_menu_item_class($classes, $item, $args) {
    return array();
}

This will result in items saying <li id="" class="">. The same mechanism can be used to set IDs to something useful, such as the page slug.

If you look in the wp_nav_menu() function, you see the items are written by walk_nav_menu_tree(), which calls Walker_Nav_Menu to do the work (unless you specified your own walker class). This class contains a method start_el() that is called for each menu item. In this function, you see that the classes are filtered through nav_menu_css_class and the id is filtered through nav_menu_item_id. So if you attach your own code to these hooks, you can change them to anything you want.

Submenus are always wrapped with <ul class="sub-menu">, the main wrapper can be changed via the menu_id and menu_class arguments.