CSS3 毛玻璃效果

body {
    min-height: 100vh;
    box-sizing: border-box;
    margin: 0;
    padding-top: calc(50vh - 6em);
    font: 150%/1.6 serif;
    background: url("iphone.jpg") fixed 0 center;
    background-size: cover;
}
main {
    margin: 0 auto;
    padding: 1em;
    max-width: 30em;
    border-radius: .3em;
    box-shadow: 0 0 0 1px hsla(0,0%,100%,.3) inset,
                0 .5em 1em rgba(0, 0, 0, 0.6);
    text-shadow: 0 1px 1px hsla(0,0%,100%,.3);
    background: hsla(0,0%,100%,.3);
}

<main>……</main>

body {
    …
    background: url("iphone.jpg") fixed 0 center;
    background-size: cover;
}
main {
    …
    background: hsla(0,0%,100%,.3);
}

main {
    …
    -webkit-filter: blur(3px);
    filter: blur(3px);
}

main {
    position: relative;
    …
}
main::before {
    content: '';
    position: absolute;
    top: 0; right: 0; bottom: 0; left: 0;
    z-index: -1;
    -webkit-filter: blur(20px);
    filter: blur(20px);
    background: rgba(255,0,0,.5);
}

main {
    …
    overflow: hidden;
}
main::before {
    …
    margin: -30px;
}