With javascript off it just stalls at "reading" forever. There are certainly some viewport properties and other things it does know even without JS execution, but the mitigation is significant. And the page itself (the JS application) cannot act on that data or communicate it. Instead it has to be processed by some other application on the backend or wherever. Not in my browser by my computer.
As an experiment, I made a small retail shop (< 30 products) that would use JS for modern style async/await calls, but would then use old school POSTs if JS was disabled with full page reloads on every POST. it sucked to dev and as UX, but it was possible to do. Had the non-JS POST style updates been any less annoying, it might have been viable. Nobody likes full reloads. They suck. JS can do nice things for UX. It's just that we can't have nice things because people suck
That's what frames are for. Only reload the frame with the important data in it (total cost, list of products in cart) and point the category links in the page to open in the same frame as the shopping cart. You can even style the frame contents with the main page's stylesheet so it only needs to load a `$41.29` total if that's all that's changed.
