Apple has a private CSS property to add Liquid Glass effects to web content

Yes it works and no, we can't

Before you, like me, fire up Safari and start editing some CSS, I have bad news: no, it doesn't work on the web. As well it shouldn't. But it also doesn't work by default in an app using WKWebView, you have to toggle a setting in WKPreferences called useSystemAppearance... and it's private. So if you use it, say goodbye to App Store approval.

Who cares?

It's an interesting piece of trivia but no-one outside of Apple can use it.

… ffs 🤦