A use case for allowing to show webxdc of static HTML + CSS in the chat with JS disabled is to provide an alternative for space-efficient and accessible stickers, animations and lightweight interactive CSS-only galleries and combinatoric logic. You can already achieve this with SVG+SMIL, but competence for creating such from scratch is high and it can’t be usually reused elsewhere.
Whereas, existing png & jpg thumbnails of a game or the current snippet from the doc or calendar app could be represented by its own existing HTML&CSS without extra design effort if the app is implemented according to progressive enhancement (i.e., shows content without JS). I agree that a thumbnail (HTML+CSS) sent with the last update would be sufficient to achieve this - no need to generate it every time the chat is opened.
Adding clickable href to this as mentioned above would also be a natural extension. It could be used both for general interaction shortcuts as mentioned for voting. It could also make the flow of generally starting an app more seamless: pressing the button on the docked app could render the same app at the same space without much apparent jumps, while possibly replacing the locked-down iframe with a JS-enabled one under the hood. Such apps could still be made fullscreen or popped out to their own window on desktop by the user.
Additionally, if we allowed JS in apps signed by DC, the user or their trusted friends, technologies from the demoscene would enable displaying of compact animations & music as stickers within mere kilobytes instead of forwarding tens of megabytes of videos around.