non-standard functionality of the latter.

My guy. In the 90's ALL browsers were non-standard. Even at the protocol level.
http/0.9 - 1991
http/1.0 - 1996
http/1.1 - 1997

html/1.0 - 1991
html/2.0 - 1995 revised in 1996, and 97.
html/3.0 - 1997
html/4.0 - 1997 revised in 1998, 99, and 2000.

Then comes all the add-ons like flash, shockwave, etc... Nothing was standard at this time-frame. We threw everything possible into browsers. Toolbars for literally everything (I remember even having winamp controls in my browser).

https://developer.mozilla.org/en-US/docs/Web/HTTP/Evolution_of_HTTP

Between 1991-1995, these were introduced with a try-and-see approach. A server and a browser would add a feature and see if it got traction.

Literally sites and browsers would just implement stuff just to implement and see if it became used.

A lot of recent times (2010's mostly) has been back peddling the mad rush of just shoving EVERYTHING into browsers. Now I actually fear we're going to far though... With google removing useful backend stuff for plugins and such. I just hope Firefox never follows suit.