在子元素滾動時保持主體bg固定

html {
  height: 100%;
  width: 100%;
}

html, body {
  margin: 0;
  padding: 0;
}

body {
  height: auto;
  width: auto;
}

html {
  background: radial-gradient(#bcd197, #325757);
  background-repeat: no-repeat;
  background-size: cover;
  background-attachment: fixed;
}

body {
  outline: blue 3px dashed;
  background-image: url("https://i.ibb.co/NFvCfrj/texture.png");
  background-repeat: no-repeat;
  background-size: cover;
  background-position: center center;
  background-attachment: fixed;
}

div {
  outline: red 3px dashed;
  height: 200rem;
  width: 10rem;
  background-color: rgba(0,0,0,.5);
  margin: 1rem;
}

<div></div>