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This was recently implemented in Chromium-based browsers (M76) too. https://web.dev/backdrop-filter has some more information about the CSS, and https://www.chromestatus.com/feature/5679432723333120 has more information about the implementation.

And this is landing very soon in Firefox, behind a pref. https://twitter.com/svoisen/status/1159322370122768384

As of yet, the browser support for this is very shoddy. Not to mention outdated/legacy browsers like IE.