Menu Link Highlighted For Current Page

I have no idea how it works, how your style sheet knows which page you are on, but I have these few lines of CSS that will highlight the page’s link in my menu if you are on that page, as far as categories or custom post type posts, but for tags and WP generate pages it doesn’t recognize it. This is the code I am using

#access .current-menu-ancestor > a,
#access .current_page_item > a,
#access .current_page_ancestor > a,
#access .current_page_parent {
	background-color: #ddd;
}