Earlier quoted context omitted.
What you’re referring to is splitting the presentation from the content. The server (eg Facebook) provides you with the content, and your computer/software displays it to your liking (ie without ads and spam and algorithmically recommended crap). There’s a lot of history around that split, and the motivation for HTML/CSS was about separating presentation from the content in many ways. For another example, once upon a…
> you could chat with a Facebook friend from your Google Hangouts account This was never true. There was an XMPP-speaking endpoint into Facebook's proprietary chat system, but it wasn't a S2S XMPP implementation and never federated with anything. It was useful for using FBChat in Adium or Pidgin, but not for talking to GChat XMPP users.
[1] https://googletalk.blogspot.com/2006/01/xmpp-federation.html