.bilibili {
  position: relative;
}

.bilibili::after {
  content: "";
  display: block;
  padding-bottom: calc(100% / (16 / 9));
}

.bilibili > iframe {
  width: 100%;
  height: 100%;
  position: absolute;
  top: 0;
  left: 0;
  border: 0;
}