.demo-video{
  width: 85vw;                 /* responsive width */
  max-width: 1280px;           /* don’t get *too* huge on 4K monitors */
}

/* Let CSS control sizing rather than the HTML attributes */
.demo-video iframe{
  width: 100%;
  height: auto;                /* works with aspect-ratio */
  aspect-ratio: 16 / 9;        /* keeps 16:9 without fixed px height   */
  border: none;                /* cosmetic: hides default frameborder */
}