Why not using marked.js or equivalent, plugging it with a Fetch API instead?

example:

  <html>
  <head><title>hello</title></head>
  <body>
  <main id=root ></main>
  <script src=./marked.min.js async defer ></script>
  <script>
  async function load() {
    let element = document.querySelector("main#root");
    await fetch("./index.md") 
      .then(resp => resp.text())
      .then(text => marked(text, { ... }))
      .then(html => {
        element.innerHTML = html;
      });
  }
  window.onload = load;
  </script>
  </body>
  </html>

since it directly injects the resulting html, you may set "unsafe HTML" option in marked (or markdown.js) and include <title> tags within your markdown document. this will _also_ rewrite the browser title(s)

> edit: formatting