This could be any page on your web site.
A button or any other clickable element launches the WMI in fullscreen mode.
Selecting "Exit Fullscreen" from the WMI's Option Menu, or using the "ESC" key or back button on a mobile device would restore the original page.
Then, clicking or touching the button again will re-open the WMI in its previous state.
This page can contain additional content of any kind including a header, a footer, navigation controls, images and text.
The "button" may be styled however you want.