True CSS neon relies on multiple text-shadow layers spreading further outwards.

The Core Neon CSS

.neon-text {
  color: #fff;
  text-shadow:
    0 0 5px #fff,
    0 0 10px #fff,
    0 0 20px var(--neon-color),
    0 0 40px var(--neon-color),
    0 0 80px var(--neon-color),
    0 0 90px var(--neon-color);
}

By allowing a user to change --neon-color via an input picker, we can create a dynamic sign generator. See the Live Demo to type your own text!