There is no point of introducing yet another standard to do something that can be done in few lines of JS.

   customElements.define(
     "html-include",
     class extends HTMLElement {
       connectedCallback() {
         fetch(this.getAttribute("href"))
           .then((response) => response.text())
           .then((text) => (this.innerHTML = text))}});

The browser has had a powerful scripting language built in as a core primitive for decades, and yet people would rather create more standards to avoid using it for ideological reasons, and then complain that there aren't enough competing browsers.