The effect you’re looking for is possible by re-using the exact same image enlarged a few times (~10), overlapped with
position:absolute with a huge blur factor, while trimming its overflow:
You can further improve this by adding a JS that simply copies the
<img> on the fly and applies class
.blur to it (so you don’t have to manually add two images to each card).
Note css-filters don’t yet have full browser support.
In the example you linked, the blur is done using SVG
<filter> (slightly better browser support) and a
<feGaussianBlur>, but the principle is the same.