如何在bootstrap轮播中添加webkit过渡？

Question

我的轮播代码与get bootstrap网站的代码基本相同。图片大小相同。

    <div class="carousel-item">
      <img class="d-block img-fluid" src="https://i.imgur.com/WBYXYze.jpg" alt="Third slide">
    </div>
  </div>
  <a class="carousel-control-prev" href="#carouselExampleIndicators" role="button" data-slide="prev">
    <span class="carousel-control-prev-icon" aria-hidden="true"></span>
    <span class="sr-only">Previous</span>
  </a>
  <a class="carousel-control-next" href="#carouselExampleIndicators" role="button" data-slide="next">
    <span class="carousel-control-next-icon" aria-hidden="true"></span>
    <span class="sr-only">Next</span>
  </a>
</div>

我的css

  .carousel-inner > .item > img, .carousel-inner > .item > a > img {
      background:linear-gradient(rgba(0,0,0,.7), rgba(0,0,0,.7))
    }  

添加此css不会对图像产生任何影响，并且控制台上没有错误。