Repeat a HTML element for multiple pages, using vanilla JS

test.html

<include src="path_to_your_html_file"></include>

<script type="text/javascript">  
  async function includeHTML() {
      const includes = document.getElementsByTagName('include');
      [].forEach.call(includes, include => {
          let filePath = include.getAttribute('src');
          fetch(filePath).then((file) => {
          file.text().then((content) => {
              include.insertAdjacentHTML('afterend', content);
              include.remove();
          });
          });
      });
  };

  includeHTML();
  
  // Using with DOM manipulation
  function waitForElm(selector) {
    return new Promise(resolve => {
        if (document.querySelector(selector)) {
            return resolve(document.querySelector(selector));
        }

        const observer = new MutationObserver(mutations => {
            if (document.querySelector(selector)) {
                resolve(document.querySelector(selector));
                observer.disconnect();
            }
        });

        observer.observe(document.body, {
            childList: true,
            subtree: true
        });
    });
    
    // And use async functions when use the document.getElementSomething...
    waitForElm('.mobile-menu').then((elm) => { });
}
</script>

<!-- Credits: W3Schools -->