This 9-minute video from Juxtopposed on blurring is a great watch. There are an awful lot of ways to blur things on the web, from filter, backdrop-filter, <feGaussianBlur>, to WebGL. I particularly like the idea of masking an element with a backdrop-filter with a gradient so, for instance, a header can fade out how much blur is being applied.
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Demystifying the View Transition Pseudo Tree
Each pseudo element plays a distinct role in how the view transition animates. The browser does most of the heavy lifting though, which makes it a little hard to see what’s actually happening under the hood.