Seems like overkill to bring in a framework just for inlining some static html. If that's all you're doing, a self-replacing script tag is neat:

    <script>
      function includeHTML(url) {
        const s = document.currentScript
        fetch(url).then(r => r.text()).then(h => {
          s.insertAdjacentHTML('beforebegin', h)
          s.remove()
        })
      }
    </script>

...

    <script>
      includeHTML('/footer.html')
    </script>

The `script` element is replaced with the html from `/footer.html`.