Before we get into extreme server side rendering (XSSR), we have to talk about normal server side rendering (SSR). This comes in two flavours, which I'm calling old-school and new-school.

Old-school SSR involves having a server which uses some logic to create the HTML of the web page on-the-fly. For example, you might hit /users/39, and it might give you the details of user 39. These details might be from a database, or they might come from somewhere else. The important part is there's no corresponding 39.html on the disk. The HTML is created dynamically by the back-end server. On the front-end side, there's no JavaScript or other logic required to render the page. As a result, once the page is loaded, there's no ability for it to be dynamic.

New-school SSR is similar to old-school SSR, but it does involve a bit of front-end JavaScript logic.