body {
  background-image: url("http://lorempixel.com/1920/1080");
  background-size: cover;
  /*Esto simula el sitio debajo del modal*/
}

.modal {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-color: rgba(0, 0, 0, 0.7);
  display: none;
}

.ventana {
  width: 360px;
  position: absolute;
  top: 8%;
  left: 50%;
  margin-left: -200px;
}

.ventana video {
  width: 100%;
  border: 3px solid #fff;
}

.ventana span {
  color: #fff;
  background-color: #000;
  padding: 0.2em 0.5em 0.3em 0.5em;
  font-family: Arial;
  font-weight: bold;
  border: 2px solid #fff;
  border-radius: 50%;
  position: absolute;
  top: 0px;
  right: 0px;
  cursor: pointer;
}