alt text on images is an important requirement for accessibility and is one of the most commonly skipped items.

img:not([alt]) {
  filter: opacity(0.5);
}
// you could also try
// border: 5px solid red inset;
// inset is used to avoid the border interfering with the layout.
// you could also try
// filter: blur(10px);					

See the Pen
eYmqMVw by Justin Avery (@justincavery)
on CodePen.