Cool idea! Since you said you were new to JavaScript, can I share some code with you:

<!-- Use Custom Elements-compatible tag name -->
<!-- Define refresh as an attribute, not a class -->
<jasna-component src="components/navbar.html" refresh="10000"></jasna-component>
<jasna-component src="components/header.html" refresh></jasna-component>
<jasna-component src="components/footer.html"></jasna-component>

<script>
  async function loadContent(component) {
    const url = component.getAttribute('src');
    const response = await fetch(url);
    // Very risky! This could lead to an XSS attack.
    component.innerHTML = await response.text();
  }

  async function liveComponents(component) {
    const refreshMs = Number(component.getAttribute('refresh'));
    const sleepTime = Number.isInteger(refreshMs) ? refreshMs : 2000;

    while (true) {
      await new Promise(r => setTimeout(r, sleepTime));
      await loadContent(component);
    }
  }

  const components = document.getElementsByTagName('jasna-component');

  for (const component of components) {
    loadContent(component);
    if (component.hasAttribute('refresh')) {
      liveComponents(component)
    }
  }
</script>