Трюки с CSS-анимациями:

@keyframes toggleOpacity {
  50% { opacity: 1; } /* Turn off */
  50.001% { opacity: 0.4; }
 
  /* Keep off state for a short period */
 
  52.999% { opacity: 0.4; } /* Turn back on */
  53% { opacity: 1; }
}

<link href='http://fonts.googleapis.com/css?family=Monoton' rel='stylesheet' type='text/css'>
<div>
  <p id="error">E<span>r</span>ror</p>
  <p id="code">4<span>0</span><span>4</span></p>
</div>

body { background-color: #111111; }
div {
  padding: 40px;
  font-size: 75px;
  font-family: 'Monoton', cursive;
  text-align: center;
  text-transform: uppercase;
  text-shadow: 0 0 80px red,0 0 30px FireBrick,0 0 6px DarkRed;
  color: red;
}
div p { margin:0; }
#error:hover { text-shadow: 0 0 200px #ffffff,0 0 80px #008000,0 0 6px #0000ff; }
#code:hover { text-shadow: 0 0 100px red,0 0 40px FireBrick,0 0 8px DarkRed; }
#error {
  color: #fff;
  text-shadow: 0 0 80px #ffffff,0 0 30px #008000,0 0 6px #0000ff;
}
#error span {
  animation: upper 11s linear infinite;
}
#code span:nth-of-type(2) {
  animation: lower 10s linear infinite;
}
#code span:nth-of-type(1) {
  text-shadow: none;
  opacity:.4;
}
@keyframes upper {
  0%,19.999%,22%,62.999%,64%, 64.999%,70%,100% {
    opacity:.99; text-shadow: 0 0 80px #ffffff,0 0 30px #008000,0 0 6px #0000ff;
  }
  20%,21.999%,63%,63.999%,65%,69.999% {
    opacity:0.4; text-shadow: none; 
  }
}
@keyframes lower {
  0%,12%,18.999%,23%,31.999%,37%,44.999%,46%,49.999%,51%,58.999%,61%,68.999%,71%,85.999%,96%,100% {
    opacity:0.99; text-shadow: 0 0 80px red,0 0 30px FireBrick,0 0 6px DarkRed;
  }
  19%,22.99%,32%,36.999%,45%,45.999%,50%,50.99%,59%,60.999%,69%,70.999%,86%,95.999% { 
    opacity:0.4; text-shadow: none; 
  }
}