That's surprising! Wasn't it Philip Wadler who said "The essence of XML is this: the problem it solves is not hard, and it does not solve the problem well."

http://harmful.cat-v.org/software/xml/

Was it far from JSX?

It was almost literally JSX (except with Scala syntax instead of Javascript, of course).

An XML literal in Scala would have been:

  // XML literals (to be dropped)
  val mails1 = for (from, to, heading, body) <- todoList yield
    <message>
      <from>{from}</from><to>{to}</to>
      <heading>{heading}</heading><body>{body}</body>
    </message>
  println(mails1)

This was replaced with "XML string interpolation", which

  // XML string interpolation
  val mails2 = for (from, to, heading, body) <- todoList yield xml"""
    <message>
      <from>${from}</from><to>${to}</to>
      <heading>${heading}</heading><body>${body}</body>
    </message>"""
  println(mails2)

VB.NET still has them. I remember doing a project with an XML database (ugh) back in college when the version with the literals was released. I was ecstatic.

ActionScript 3 as well as the E4X support in Mozilla's Browser+JS/XUL engine for a while (removed now afaik).

Around that time it was pretty nice passing around XML, as I was forced to work with VB.Net which also had an XML literal syntax on the backend and Flash/AS3 on the UI.

I had built a POC with E4X that was VERY similar to React/Redux over a decade before React, but the other browser vendors didn't have it... At the time IE and Chrome were shifting towards JSON.

The irony, to me, is how it is viewed as a bit of a mistake that they had XML in Scala. Hard to square that with how instrumental JSX was in getting some of the modern JavaScript frameworks as far as it has.

If the web spec had a JSON analog for HTML, I think you'd see a lot less love for JSX.

Maybe? Though, I suspect JSX just got lucky with when it got popular. There were plenty of mixed HTML/Script things in the past. Most of them met a lot of resistance.