html,
body {
  background-color: black;
  color: white;
  font-family: Verdana, sans-serif;
  text-align: center;
  margin: 0;
  padding: 0;
}

/* Desktop / default */
#bigSquare3{
  position: relative;
  width: 100%;
  max-width: 900px;
  height: 550px;      /* bigger desktop box */
  margin: 0 auto;
  overflow: hidden;
  background: black;
}

#bigSquare3 iframe{
  position: absolute;
  inset: 0;           /* top/right/bottom/left: 0 */
  width: 100%;
  height: 100%;
  border: 0;
  display: block;

  /* desktop zoom */
  transform: scale(1.25);
  transform-origin: 50% 18%;
}

/* Mobile: slightly MORE zoom + different focus point */
@media (max-width: 600px){
  #bigSquare3{
    height: 300px;
  }

  #bigSquare3 iframe{
    transform: scale(1.25);
    transform-origin: 50% 22%;
  }
}