Perfect Inner Shadows

<feFlood x="-5%" y="0" height="100%" width="110%" flood-color="#222" result="COLOR-black" />

Inner Shadow

<feMerge result="source-on-black">
  <feMergeNode in="shadow-bg" />
  <feMergeNode in="SourceGraphic" />
</feMerge>

Inner Shadow

<feGaussianBlur in="source-on-black" stdDeviation="2" result="blur"/>

Inner Shadow

<feOffset in="blur" dx="0" dy="2" result="offset" />
<feComposite operator="in" in="offset" in2="SourceGraphic" />

Inner Shadow