Most websites will not need to create their own page elements because the built-in Page class should provide everything you need for most situations. If you do want to do something unusual with a page or set of pages on your class then you can either inherit from the built-in Page class or implement the IPage interface to fully customize how pages are rendered.
This is an advanced topic not recommended for most application developers, and any code you write using custom page elements is not guaranteed to work in future versions of this framework.