I made flickering neon text with only CSS, and here's how!
It was pretty easy, I made an animation that changes the opacity pseudo-randomly, here are the keyframes:

@keyframes flicker {  
  0% { opacity: 100%; }  

  10% { opacity: 100%; }  
  11% { opacity: 0%; }  
  12% { opacity: 100%; }  
  13% { opacity: 0%; }  
  14% { opacity: 100%; }  

  30% { opacity: 100%; }  
  31% { opacity: 0%; }  
  32% { opacity: 100%; }  

  50% { opacity: 100%; }  
  51% { opacity: 0%; }  
  52% { opacity: 100%; }  

  70% { opacity: 100%; }  
  71% { opacity: 0%; }  
  73% { opacity: 0%; }  
  74% { opacity: 100%; }  


  90% { opacity: 100%; }  
  91% { opacity: 0%; }  
  92% { opacity: 100%; }  

  100% { opacity: 100%; }  
}

And different colors have different delays and duration.

That's all, Thanks for looking!