Like hx-boost or Tuborlinks. Just 3.2 kB minified+gzipped. Optional `data-no-morph`

spa-for-mpa.html

<script src="https://unpkg.com/[email protected]"></script>
<script>
const morphTo = async (url, opts = {}) => {
  const res = await fetch(url, { headers: { 'X-Requested-With': 'Idiomorph' }, ...opts });
  const text = await res.text();
  const html = new DOMParser().parseFromString(text, 'text/html');
  Idiomorph.morph(document.body, html.body, { restoreFocus: true });
};

addEventListener('click', e => {
  const a = e.target.closest('a');
  if (a && !a.hasAttribute('data-no-morph') && a.origin === location.origin && a.href !== location.href) {
    e.preventDefault();
    morphTo(a.href);
    history.pushState(null, '', a.href);
  }
});

addEventListener('submit', e => {
  const f = e.target.closest('form');
  if (f && !f.hasAttribute('data-no-morph')) {
    e.preventDefault();
    const fd = new FormData(f);
    const method = (f.method || 'GET').toUpperCase();
    const url = f.action || location.href;
    if (method === 'GET') {
      const q = new URLSearchParams(fd).toString();
      morphTo(url + '?' + q);
      history.pushState(null, '', url + '?' + q);
    } else {
      morphTo(url, { method, body: fd });
    }
  }
});

addEventListener('popstate', () => morphTo(location.href));
</script>