A quick and simple paint worklet to draw borders inside the element. Use it with arguments instead of custom properties to apply many inner borders to one element

paint(inner-border, <width>, <color>, <offset>, <angle>)

Support: Chrome with flags (paint worklet with arguments)

Forget the fat lady! You're obsessed with the fat lady! Drive us out of here! Did he just throw my cat out of the window? Forget the fat lady! You're obsessed with the fat lady! Drive us out of here! Life finds a way. What do they got in there? King Kong?
.el--1 {
  background: paint(inner-border, 2px, white, 10px, 0deg),
             url(https://picsum.photos/256/256?image=62);
}
.el--2 {
  background: paint(inner-border, 2px, white, 10px, 0deg),
             paint(inner-border, 1px, white, 14px, 0deg),
             url(https://picsum.photos/256/256?image=154);
}
.el--3 {
  background: paint(inner-border, 2px, black, 10px, -2deg),
             paint(inner-border, 2px, black, 10px, 0deg),
             paint(inner-border, 2px, black, 10px, 3deg),
             url(https://picsum.photos/300/300?image=201);
}
.el--4 {
  background: url(https://picsum.photos/256/256?image=14);
  -webkit-mask-image: paint(inner-border, 5px, white, 10px, 0deg), linear-gradient(black, black);
  -webkit-mask-composite: xor;
  mask-image: paint(inner-border, 5px, white, 10px, 0deg), linear-gradient(black, black);
  mask-composite: exclude;
}
.el--5 {
  background: paint(inner-border, 2px, white, 10px, 0deg) #2785b8;
}
.el--6 {
  background: paint(inner-border, 10px, white, 0, -10deg),
             url(https://picsum.photos/256/256?image=41);
}