<div class="form">
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</div>

body {
  display: flex;
  width: 100vw;
  height: 100vh;
  margin: 0;
  padding: 0;
  background-image: url('https://picsum.photos/id/129/1920/1080');
  background-attachment: fixed;
  background-position: center center;
  background-size: cover;
}
.form {
  padding: 20px;
  box-sizing: border-box;
  display: flex;
  width: 40vw;
  height: 50vh;
  position: absolute;
  top: 25vh;
  left: 50vw;
  background-attachment: fixed;
  background-position: center center;
  background-image: url('https://picsum.photos/id/129/1920/1080/?blur=10&grayscale');
  border: 1px solid white;
  border-radius: 5px;
  background-size: cover;
}