一个position为fixed的div，宽高自适应，怎样让它水平垂直都在窗口居中？

.div{
    position: fixed;
    left: %;
    top: %;
    -webkit-transform: translate(-%, -%);
    transform: translate(-%, -%);
}

<div id="flex-wrap">
    <div class="x"></div>
</div>
style
#flex-wrap {
  width: 1000px;
  height: 300px;
  border: 1px solid red;
  margin:  auto;
  display: flex;
  flex-direction: row;
  flex-wrap: wrap;
  justify-content: center;
  align-items: center;
}
.x {
  width: 100px;
  height: 100px;
  background: red;
}