Such as reading a book in offline mode, or listening to a sermon in offline mode. When using a static site generator it will a different kind of experience that would enable offline media. It would require to parse the metadata out of the HTML or to include it twice in the markup of the pages, once as HTML and once as JSON. This itself is doable.
It is possible to create TypeScript that will only work when offline. It is technically possible to create an offline experience similar to LightNet version 1 but the rendering mechanism of the page would be duplicated. React could be used but then the App has to be carefully chunked up into parts so that it is possible to only ship that part to the front-end so that it can be used when using the app offline.
Searching would than be working on top of the cache of the HTML pages or on the JSON.
My recommendation is to include the meta data also as JSON. When this turns out to be too much data a performance tweak can be made to parse it out of the HTML.
The challenge I see here is that testing will become much harder, the online and the offline experience are then driven by different code bases.