I love that as well, I hate bloated sites just as the next one here.
But for any more complex form with dynamic content, JS is still inevitable. I guess for HN the input form at the bottom is enough. But Algolia uses a custom input component already.
On reflection my earlier comment was a little off the mark - all 3 of those sites use JavaScript, but minimally.
Zero JavaScript is rarely practical when there's meant to be interactivity of any kind. The issue isn't the language, but the appalling bloat that is now the norm on the web. And, course, bad design in its various forms.
But for any more complex form with dynamic content, JS is still inevitable. I guess for HN the input form at the bottom is enough. But Algolia uses a custom input component already.