YouTube video into div wrapper to make it responsive. Plain JS solution.

video-wrap.css

.videoWrapper {
  position: relative;
  padding-bottom: 56.25%; /* 16:9 */
  height: 0;
}
.videoWrapper iframe {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
}

video-wrap.js

const wrapVideos = () => {
  const iframeVideos = document.querySelectorAll('iframe[src*="youtube.com/embed/"]');
  if (iframeVideos) {
    for (const [i, v] of iframeVideos.entries()){
      let original = v;
      let wrapper = document.createElement('div');
      wrapper.classList.add(`videoWrapper`, `video--${i}`);
      wrapper.appendChild(original.cloneNode(true));
      original.replaceWith(wrapper);
    }
  }
}

wrapVideos();