Make neon glowing button with CSS only.

neon-button.css

/* <button class="neon-button">Neon Button</button> */

:root {
  --clr-neon: hsl(317 100% 54%);
  --clr-bg: hsl(323 21% 16%);
}

*::before,
*::after {
  box-sizing:border-box;
}
body {
  min-height: 100vh;
  display: grid;
  place-items: center;
  background: var(--clr-bg);
  font-family: "Balsamiq Sans", cursive;
  color: var(--clr-neon);
  padding-right: 10rem;
}

.neon-button {
  background: transparent;
  color: var(--clr-neon);
  font-size: 4rem;
  display: inline-block;
  cursor: pointer;
  text-decoration: none;
  border: var(--clr-neon) 0.125em solid; 
  padding: 0.25em 1em;
  border-radius: 0.25em;
  
  
  text-shadow:
    0 0 0.125em hsl(0 0% 100% / 0.5 ),
    0 0 0.45em var(--clr-neon);
  
  box-shadow:
    inset 0 0 0.5em 0 var(--clr-neon),
    0 0 0.5em 0 var(--clr-neon);
  position: relative;
  transition: background-color 100ms linear;
    
}

.neon-button::before {
  pointer-events: none;
  content: '';
  position: absolute;
  background: var(--clr-neon);
  top: 120%;
  left: 0;
  width: 100%;
  height: 100%;
  
  transform: perspective(1em) rotateX(40deg) scale(1, 0.35);
   
  filter: blur(1em);
  opacity: 0.7;
}

.neon-button::after {
  content: '';
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  box-shadow: 0 0 2em 0.5em var(--clr-neon);
  background: var(--clr-neon);
  z-index: -1;
  opacity: 0;
  transition: opacity 100ms linear;
}

.neon-button:hover,
.neon-button:focus {
  color: var(--clr-bg);
  text-shadow: none;
}

.neon-button:hover::before,
.neon-button:focus::before{
  opacity: 1;
}

.neon-button:hover::after,
.neon-button:focus::after{
  opacity: 1;
}