Looks like "backdrop-filter: blur(<length>)" would be the way to achieve the effect shown... but it's not supported by anything yet:

https://developer.mozilla.org/en-US/docs/Web/CSS/backdrop-fi...

But if you want to blur a whole element (eg, your main page content while a modal is open) instead of just the part behind another one, that would be "filter: blur(<length>)" - and works in all modern browsers except IE (even my Android Firefox displays the demos on this page correctly):

https://developer.mozilla.org/en/docs/Web/CSS/filter#blur()_...

And IIRC there's some non-standard way to achieve the same effect in IE.