Result

css

html,body{
  margin:0;
  height:120%;

}
.header{/*ヘッダ部分*/
  position:relative;
  overflow:hidden;
  display:flex;
  flex-wrap: wrap;
  justify-content: center;
  align-items: flex-start;
  align-content: flex-start;
  height:50vw;
  min-height:400px;
  max-height:550px;
  min-width:300px;
  color:#eee;
}
.header:after{/*下部にうっすらグラデーションで影を作る*/
  content:"";
  width:100%;
  height:40%;
  position:absolute;
  bottom:0;
  left:0;
  z-index:-1;
 background: linear-gradient(to bottom, rgba(0,0,0,0) 0%,rgba(27,32,48,1) 100%);
}
.header:before{/*疑似要素に背景画像を指定、常時アニメーションさせつつ、スクロール時は固定*/
  content:"";
  width:100%;
  height:100%;
  position:absolute;
  top:0;
  left:0;
    -webkit-backface-visibility: hidden;
    -webkit-transform: translateZ(0) scale(1.0, 1.0);
    transform: translateZ(0);
  background:#1B2030 url(https://unsplash.it/1999/999?image=1063) top center no-repeat;
  background-size:cover;
  background-attachment:fixed;
  animation: grow 60s  linear 10ms infinite;
  transition:all 0.2s ease-in-out;
  z-index:-2
}


@keyframes grow{/*微差で拡大/縮小するアニメーション設定*/
  0% { transform:scale(1)}
  50% { transform:scale(1.2)}
}
.content{  
  padding:5% 10%;
  text-align:justify
}

html

via

CSS Animated Header