/* spec 6.1. */
@keyframes shake {
  0%, 100% { left: 0; }
  14.29%, 42.86%, 71.43% { left: -3px; }
  28.57%, 57.14%, 85.71% { left: 3px; }
}

/* spec 4.3. */
@keyframes blink {
  0%, 100% { opacity: 1; }
  50% { opacity: 0; }
}