https://plan.polsl.pl/ courses filter

filter.js

(() => {
  const dd = $$;
  const d = $;

  const iframe = d('frame[name="page_content"]').contentWindow.document;
  const courses = dd('.coursediv', iframe);

  const filterElements = (e) => {
    courses.forEach(course => {
      if (course.innerText.includes(e.target.value)) {
        course.style.display = 'block';
      } else {
        course.style.display = 'none';
      }
    })
  };

  const input = document.createElement('input');
  input.style = 'position: absolute; top: 10px; left: 10px; width: 400px; height: 50px; border: 2px solid black;';
  input.onchange = filterElements;

  const html = document.getElementsByTagName('html')[0];
  html.appendChild(input);
})();