/*
optical illusion
The horizontal bar in the center is a solid gray color, but it looks likee it follows a gradient opposite of the one in the background
*/
body {
  margin: 0;
  height: 100vh;
  overflow: hidden;
}

body::before {
  content: "";
  position: absolute;
  top: 50%;
  left: 50%;
  translate: -50% -50%;
  width: 110vmax;
  height: 110vmax;
  background: repeating-linear-gradient(90deg, #ddd 15vmin, #222 30vmin, #eee 45vmin);
  filter: blur(1vmax)
}

body::after {
  content: "";
  position: absolute;
  top: 50%;
  left: 50%;
  translate: -50% -50%;
  width: 100%;
  height: 4vmin;
  background: #777;
  transition: height 1s
}

body:hover::after {
  height: 100%;
}