rivers, contextual word splits, compound noun splits, awkward pauses, there is so many downsides to mindless text justification. Only editorialised and hand crafted justified text is nice. Everything else ends up forceful and ugly.

Strange that online it fell out of favor, because text justification was the default for centuries. It's still around in PDF though, e.g. in research papers.

When there was a person physically typesetting every line, the justification wasn't generally "mindless"

I suspect the print typesetting has largely been automatized for several decades.

Is there a difference between the rules TeX uses and HTML renderers?

AIUI, yes, most browsers use greedy line breaking by default. E.g., FF does not do Knuth style[1].

This led to the CSS `text-wrap` property, and `text-wrap: pretty`[2], but pretty is not supported in FF yet, and the value `pretty` doesn't mandate Knuth's algorithm.

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=630181

[2]: https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/P...