<div class="container">
    <h1 class="neonText pulsate">
          404
    </h1>
    <h2 class="neonText pulsate">Page Not Found</h2> 

 </div>
  .neonText {
    color: #fff;
    text-shadow:
      0 0 7px #fff,
      0 0 10px #fff,
      0 0 21px #fff,
      0 0 42px #0fa,
      0 0 82px #0fa,
      0 0 92px #0fa,
      0 0 102px #0fa,
      0 0 151px #0fa;
  }


/* Additional styling */
  
body {
  font-size: 18px;
  font-family: "Helvetica Neue", sans-serif;
  background-color: #010a01;
}  

h1, h2 {
  text-align: center;
  text-transform: uppercase;
  font-weight: 400;
}
  
h1 {
    font-size: 4.2rem;
}

.pulsate {
     animation: pulsate 1.5s infinite alternate;     
}
  
h2 {
    font-size: 1.8rem;
}

.container {
  margin-top: 20vh;
}

@keyframes pulsate {
    
  100% {

      text-shadow:
      0 0 4px #fff,
      0 0 11px #fff,
      0 0 19px #fff,
      0 0 40px #0fa,
      0 0 80px #0fa,
      0 0 90px #0fa,
      0 0 100px #0fa,
      0 0 150px #0fa;
  
  }
  
  0% {

    text-shadow:
    0 0 2px #fff,
    0 0 4px #fff,
    0 0 6px #fff,
    0 0 10px #0fa,
    0 0 45px #0fa,
    0 0 55px #0fa,
    0 0 70px #0fa,
    0 0 80px #0fa;

}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.