HTML has that built in: https://developer.mozilla.org/en-US/docs/Web/Web_Components/...

You'd need some JS to make use of them though.

This honestly looks like a huge PITA compared to even the worst static site generator’s syntax. Is this actually supposed to be used as end-developer templates?

True, but keeping to the spirit of the comment I replied to, I’d prefer it to be HTML only. Something like <template src=“header.html”> and it just uses a relative or absolute path.

Static site generators like Jekyll and Hugo are perfect for this :)

You can run React (and everything else) off a CDN.

React runs in the browser. You probably mean serve the files off a CDN

Yeah that's what I meant.

It can also run in the server with Server Side Rendering[0]. I believe some CDN's also offer support for this, too.

[0]: https://reactjs.org/docs/react-dom-server.html