font-awesome-animation/scss/flash.scss

$faa-flash-speed: 2s;
@keyframes flash {
  0%, 50%, 100% {
    opacity: 1;
  }
  25%, 75% {
    opacity: 0;
  }
}
.faa-flash.animated,
.faa-flash.animated-hover:hover,
.faa-parent.animated-hover:hover > .faa-flash {
  animation: flash $faa-flash-speed ease infinite;
}
.faa-flash.animated.faa-fast,
.faa-flash.animated-hover.faa-fast:hover,
.faa-parent.animated-hover:hover > .faa-flash.faa-fast {
  animation: flash ($faa-flash-speed * $faa-speed-coeff-fast) ease infinite;
}
.faa-flash.animated.faa-slow,
.faa-flash.animated-hover.faa-slow:hover,
.faa-parent.animated-hover:hover > .faa-flash.faa-slow {
  animation: flash ($faa-flash-speed * $faa-speed-coeff-slow) ease infinite;
}