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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Simulating Crop Marks
Crop marks are an idea that comes from the print design world. Design in the bleed area will be cut away by giant cutter machines, and that bleed area is designated by the crop marks. We can do it on the web too, just for kicks.