<div class="form">
		<div class="blur">
	</div>
	Lorem ipsum dolor sit amet, consectetur adipisicing elit. Aut sequi cum ipsa suscipit voluptatibus perferendis beatae, est voluptates iste assumenda quod harum unde voluptatem alias sint quam. Magnam, aspernatur quod?
</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: 0 0;
	background-size: cover;
	background-repeat: no-repeat;
}
.form {
	transition: all .1s ease;
	padding: 20px;
	box-sizing: border-box;
	display: flex;
	width: 40vw;
	height: 50vh;
	position: absolute;
	top: 25vh;
	left: 50vw;
	background-color: rgba(255,255,255,.2);
	border: 1px solid white;
	border-radius: 5px;
	overflow: hidden;
}
.blur {
	position:absolute;
	background-attachment: fixed;
	background-position: 0 0;
	background-size: cover;
	background-repeat: no-repeat;
	
	background-image: url('https://picsum.photos/id/129/1920/1080/?');
    filter: blur(4px) grayscale(50%);
    height: 110%;
    width: 110%;
    top: -5%;
    left: -5%;
    z-index: -1;
}

External CSS

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External JavaScript

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