CSS动画覆盖

div.relative {
  position: relative;
  width: 400px;
  height: 200px;
  border: 3px solid #73AD21;
}

div.absolute {
  position: absolute;
  top: 50%;
  right: 0;
  width: 200px;
  height: 100px;
  border: 3px solid #73AD21;
  opacity: 1;
  background-color: blue;
}

.overlay {
  position: fixed;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  background: rgba(0, 0, 0, 0.95);
  color: white;
  z-index: 1;
}

.div1 {
  animation: 750ms 1 forwards ani;
}

@keyframes ani {
  0% {
    opacity: 0;
  }
  100% {
    opacity: 1;
  }
}

<h2>
  position: absolute;</h2>

<p>An element with position: absolute; is positioned relative to the nearest positioned ancestor (instead of positioned relative to the viewport, like fixed):</p>

<div class="relative">
  This div element has position: relative;
  <div class="div1">
    This is a div
    <div class="overlay">
      This div element has position: fixed;
    </div>
  </div>

  <div class="absolute">
    This div element has position: absolute;
  </div>
</div>