This page can rewrite itself inside a ServiceWorker behind this website. Try adding a couple of DOM nodes using a button below, then reload the page. Notice that the page arrives with new DOM nodes to the browser, thus there is no blinking / initial pause. If you inspect the fetched page's source in the Network tab you will see the updated HTML on each reload.
The self-rewriting portion is below
Load the page first - I assume you have already done this, if you are reading this
Click "Add ..." button several times, you should see new items in the list.
Reload the page - you should see the items you have added right away - they are part of the loaded page, not added dynamically later. You can confirm this by inspecting the downloaded page's source or the contents of the page in the Network tab.
Open browser console to see any messages / errors