Comment by mg

At the core, this seems to be about shadowrootmode=open, a new html feature which already landed in Chromium and Safari:

    <div>
      <template shadowrootmode=open>
        This is <slot name=a></slot> and <slot name=b>
      </template>
      <span slot=b>sunny</span>
      <span slot=a>funny</span>
    </div>

renders as:

    This is funny and sunny

So it looks like HTML got a bit of a native template system now.

What I have been wanting for decades is a native template system in HTML which supports urls. Similar the the script tag, but which loads html:

    <html url="/header.html">
    <html url="/content.html">
    <html url="/footer.html">

I always wondered why this was never implemented. It seems such a basic functionality which would allow to comfortably build websites in a modular way without scripts or serverside processing.