How was the web before React.js?

The web? Mostly the same. React is close to irrelevant at this zoom level.

Front-end web development? Not much, IMO. It didn't realy bring anything revolutionary. The field started to suck way before React.

What do you mean by before React.js? Which year? in particular? I do a brief rant about web development from 1998 up today http://minid.net/2019/04/07/the-css-utilitarian-methodology/. You can have an idea with that.

Does anybody remember the time when JS template libraries like Backbone, Handlebars, Emberjs was briefly popular after jQuery but before React/JSX came around. I think this was the early 2010s.

We had a few years of Angular with jQuery and before that it was very much only jQuery. jQuery was loved because it provided a common and good API across web browsers. Some people used backbone js but that was rare.

Before jQuery, the most fancy projects used prototypejs and http://script.aculo.us/

Before these, I was not doing web development so I prefer to not say anything.

You are having a great ability to see a typical website w/o React. You can even enjoy of tables instead of divs.

It was so bad I don't even want to talk about it...

And it was slow, too. Back then, React was a significant speedup as well as major DX improvement.

(I'm speaking about web apps such as CRM or ERP)

jQuery. jQuery everywhere.

Better.