With a simple texteditor written page: How to define the space for opening a new HTML document within the same space on a page, and how to retrieve and place it there? That would be very useful. It could be the same style and type of content already defined, kind of having multiple subpages and leaf through. And as a way to open an external menu sheet it would solve the one-menu-for-all-(changing)-pages question of static websites.
see also: Links in HTML documents under “12.1.3 Specifying anchors and links”