JSFiddle - React, Tailwind, and code Playground
by Wolfsilver
HTML
<div style="width:100px;height:100px;border:4px solid black;box-shadow:inset 0 0 30px black;margin:10px;"></div>
<svg>
<defs>
<filter id="inset-shadow" >
<feComponentTransfer in="SourceGraphic" result="inset-selection">
<feFuncA type="discrete" tableValues="0 1 1 1 1 1"/>
</feComponentTransfer>
<feComponentTransfer in="SourceGraphic" result="original-no-fill">
<feFuncA type="discrete" tableValues="0 0 1"/>
</feComponentTransfer>
<feColorMatrix type="matrix" in="original-no-fill" result="new-source-alpha"
values="0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
0 0 0 1 0"
/>
<feGaussianBlur in="new-source-alpha" result="blur" stdDeviation="5" />
<feGaussianBlur in="new-source-alpha" result="blur2" stdDeviation="10" />
<feGaussianBlur in="new-source-alpha" result="blur4" stdDeviation="10" />
<feGaussianBlur in="new-source-alpha" result="blur5" stdDeviation="10" />
<feGaussianBlur in="new-source-alpha" result="blur3" stdDeviation="15" />
<feGaussianBlur in="new-source-alpha" result="blur6" stdDeviation="15" />
<feGaussianBlur in="new-source-alpha" result="blur7" stdDeviation="15" />
<feGaussianBlur in="new-source-alpha" result="blur8" stdDeviation="15" />
<feMerge result="blur">
<feMergeNode in="blur" mode="normal"/>
<feMergeNode in="blur2" mode="normal"/>
<feMergeNode in="blur3" mode="normal"/>
<feMergeNode in="blur4" mode="normal"/>
<feMergeNode in="blur5" mode="normal"/>
<feMergeNode in="blur6" mode="normal"/>
<feMergeNode in="blur7" mode="normal"/>
<feMergeNode in="blur8" mode="normal"/>
</feMerge>
...