css如何实现一个元素高度固定宽度按比例显示？

.content {
      margin: 0 auto;
      height: 79vh;
      .video_box {
        position: relative;
        height: 100%;
        overflow: hidden;
        margin: 0 auto;
        width: 79vh*1.778;
        max-width: 100vw;
        .ad_pic {
          display: none;
          position: absolute;
          top: 0;
          left: 0;
          width: 100%;
          height: 100%;
          background: url(6.jpg) no-repeat center;
          background-size: 100% 100%;
          img {
            width: 100%;
            height: 100%;
          }
          .btn_play {
            display: inline-block;
            .width(50);
            .height(50);
            position: absolute;;
            left: 50%;
            top: 50%;
            .margin-left(-25);
            .margin-top(-25);
            background: url(../../public/img/icon_play.png) no-repeat center;
            background-size: 100% 100%;
          }
        }
        iframe, object, embed, video {
          position: absolute;
          top: 0;
          left: 0;
          width: 100%;
          height: 100%;
        }
        .video_card {
          position: absolute;
          top: 0;
          left: 0;
          width: 2px;
          height: 1px;
        }
      }
    }

 <div class="content">
        <div class="video_box">
            <div class="ad_pic">
                <span class="btn_play"></span>
            </div>
            <video id="ad_video"
                   autobuffer
                   src="a.mp4"
                   autoplay
                   preload
                   controls=""
                   loop
                   poster="6.jpg"
                   webkit-playsinline="true"
                   playsinline="true"
                   x-webkit-airplay="allow"
                   x5-playsinline
                   x5-video-player-type="h5"
                   x5-video-player-fullscreen="true"
                   x5-video-orientation="portrait"
                   >
            </video>
        </div>
    </div>