CSS: Workaround to backdrop-filter?

As of Chrome M76, backdrop-filter is now shipped, unprefixed, and without a needed flag.

https://web.dev/backdrop-filter/

NOTE: (since this answer keeps getting downvoted because Mozilla hasn’t yet shipped it): this feature is available in Safari, Chrome, and Edge, but not yet in Firefox. Mozilla is planning to ship it very soon, but hasn’t yet. So this answer doesn’t contain a “workaround” but simply more information about which browsers require a workaround. Which still seems like useful information.

I use this to get the popular frosted glass effect. Until someone successfully invents a good polyfill for backdrop-filter I'm using a slightly transparent background as a fallback:

/* slightly transparent fallback */
.backdrop-blur {
  background-color: rgba(255, 255, 255, .9);
}

/* if backdrop support: very transparent and blurred */
@supports ((-webkit-backdrop-filter: none) or (backdrop-filter: none)) {
  .backdrop-blur {
    background-color: rgba(255, 255, 255, .5);
    -webkit-backdrop-filter: blur(2em);
    backdrop-filter: blur(2em);
  }
}

The filter will work in currently supported browsers. (Safari and Chrome with experimental Web Platform features enabled) The code should also work in future browsers that support unprefixed backdrop-filter if the spec doesn't change before that.

Examples without and with backdrop-filter support:

Update as of 5/01/22:

Backdrop Filter has received further support, check the link for the versions of each browser that support it and a quick tutorial.

Syntax:

.element {
            backdrop-filter: <filter-function> [<filter-function>]* | none
         }

Older Solution

I don't know if you're still chasing this question, but for other users in future:

This effect can be accomplished as follows with CSS Pseudo-Classes, no JavaScript is necessary! shown below:

body,
main::before {
  background: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/80625/tree.jpg) 0 / cover fixed;
}

main {
  margin: 100px auto;
  position: relative;
  padding: 10px 5px;
  background: hsla(0, 0%, 100%, .3);
  font-size: 20px;
  font-family: 'Lora', serif;
  line-height: 1.5;
  border-radius: 10px;
  width: 60%;
  box-shadow: 5px 3px 30px black;
  overflow: hidden;
}

main::before {
  content: '';
  margin: -35px;
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  filter: blur(20px);
  z-index: -1;
}

<main>
  <blockquote>"The more often we see the things around us &#45; even the beautiful and wonderful things &#45; the more they become invisible to us. That is why we often take for granted the beauty of this world: the flowers, the trees, the birds, the clouds &#45;
    even those we love. Because we see things so often, we see them less and less."
    <footer>&mdash;
      <cite>
        Joseph B. Wirthlin
      </cite>
    </footer>
  </blockquote>
</main>

Live example can be seen on Codepen: https://codepen.io/jonitrythall/pen/GJQBOp

Quick Note:

Depending on the structure of your HTML document, your z-indexes may be different to the one described in the example.