SOURCE

<div class="container">
  <div class="box">
    <p>backdrop-filter: blur(10px)</p>
  </div>
</div>

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

body {
    background-image: url('https://lorempixel.com/400/200/');
    background-position: center center;
    background-repeat: no-repeat;
    background-size: cover;
}

.container {
    display: flex;
    align-items: center;
    justify-content: center;
    height: 100%;
    width: 100%;
}

.box {
    background-color: rgba(255, 255, 255, 0.3);
    border-radius: 5px;
    font-family: sans-serif;
    text-align: center;
    line-height: 1;
    backdrop-filter: blur(10px);
    -webkit-backdrop-filter: blur(10px);
    max-width: 50%;
    max-height: 50%;
    padding: 20px 40px;
}