https://developer.mozilla.org/en-US/docs/Web/HTML/Element/po...
https://en.wikipedia.org/wiki/Server_Side_Includes
Also, other than to satisfy a purist desire, why do you need this?
You can pre-render HTML quickly using a variety of template systems, and it would outperform what you are suggesting every time (from a client's perspective). I mean, think about the potential CSS resolution complexity, FOUC, etc.
https://caddyserver.com/docs/modules/http.handlers.templates
The "include" function should do the job:
{{include "path/to/file.html"}}
https://nginx.org/en/docs/http/ngx_http_ssi_module.html
Caddy chose not to, I guess?
https://caddy.community/t/caddy-update-on-ssi-server-side-in...
Or serializing `<template>`s without needing JS. Like in lists (`<ol>`, `<ul>`, `<dl>`...). All in all `<template>`s are a nice thing to have, and I do use them a lot (like in my own portfolio), but it feels really lacking without JS.