Cool! Also, carrying forward the post author's tongue-in-cheek humour... I see your lua and raise you ~350 lines of Bash (excludes HTML templates). Take a good hard look at this shite [0]. Have you seen anything that is more obviously HTML-about-to-be-expanded-into-a-full-page?

    shite_template_standard_page_wrapper() {
        cat <<EOF
    <!DOCTYPE html>
    <html lang="en" prefix="og: https://ogp.me/ns#">
      $(shite_template_common_head)
      <body>
        <div id="the-very-top" class="stack center box">
          $(cat -)
          $(shite_template_common_footer)
        </div>
      </body>
    </html>
    EOF
    }

And, unlike PHP or whatever... have you ever seen more composable templating, as in functional programming?

    cat ./sample/hello.html |
      shite_template_common_default_page

    # Or..

    cat ./sample/hello.org |
      pandoc -f org -t html |
      shite_template_common_default_page

Just imagine all the pipelined pre/post-processor possibilities.

[0] project codename `shite`, literally, renders my site https://github.com/adityaathalye/shite

How about one line of POSIX sh with cpp? Though it uses the different approach of SSI instead of templating:

  $ cat <<EOF | sed -E 's/^[\t ]*<!--(#.*)-->$/\1/; t; s/^/\a/' | cpp -traditional -nostdinc -P -C | sed -E 's/^\a//'
  <!DOCTYPE html>
  <html>
    <head><title>My site</title></head>
    <body>
      <!--#include "navbar.inc.html"-->
      <p>Foo</p>
    </body>
  </html>
  EOF
  <!DOCTYPE html>
  <html>
    <head><title>My site</title></head>
    <body>
  <header>
    <a href="/">My cool site</a>
    <nav>
      <ul>
        <li><a href="/blog.html">Blog</a></li>
        <li><a href="/about.html">About</a></li>
      </ul>
    </nav>
  </header>
      <p>Foo</p>
    </body>
  </html>

With a little elbow grease, you even get incremental rebuild: https://git.sr.ht/~q3cpma/html-cpp

That is... a cool trick! I don't know C, so it would have never occurred to me. (Let's be honest; it would have not occurred to me even if I knew C :))

What I like about the Bash + Heredocs + substitutions approach is that it looks and feels declarative to me. Template "expansion" is just in-place substitutions (parameter expansion, shell substitution, process substitution); no string munging needed to "place" or "inject" content in the HTML tree.

Anyway, I spent way too much time figuring that out, and it works well for me, so I'm going to just roll with the sunk costs :D

Thanks for sharing your approach!