The core of this is of course the response header "Content-Type: multipart/x-mixed-replace" which in theory lets the server push updated chunks of any content type (although apparently only images in Chrome since 2013 [1]).

While it's a crude and handy way to update some content after the initial load without JS, it was also widely used with JS before Web Sockets - to push messages to clients with less latency than XHR polling.

[1] https://en.wikipedia.org/wiki/Multipart/x-mixed-replace#Mixe...