rmcg.dev/projects/random/neon-text/style.css

:root {
    --clr-neon: rgb(165, 224, 16);
    --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 {
    color: var(--clr-neon);
    font-size: 4rem;
    display: inline-block;
    cursor: pointer;
    text-decoration: none;
    /* Using em here to make border responsive to text size */
    border: var(--clr-neon) 0.125em solid;
    /* Using em here to make padding responsive to text size */
    padding: 0.25em 1em;
    /* Using em here to make border-radius responsive to text size */
    border-radius: 0.25em;

    /* this starts to give it the glow, you could just go with
    a shadow that is the same color, but better to mix it a little

    ! NOTE: IF YOU MISS that second % in the hsl it breaks it!!!
     */
    text-shadow: 0 0 0.125em hsl(0 0% 100% / 0.3), 0 0 0.45em currentColor;

    /* following makes the text backlit, better something else
    note current color isn't needed, it is inherited */
    /* box-shadow: 0 0 0.5em 0 currentColor; */
    /* inset adds the shadow to the inside of the element */
    box-shadow: inset 0 0 0.5em 0 var(--clr-neon), 0 0 0.5em 0 var(--clr-neon);

    position: relative;
}

.neon-button::before {
    position: absolute;
    content: "";
    pointer-events: none;

    background-color: var(--clr-neon);
    /* could use following or ...*/
    /* top: 0;
    right: 0;
    bottom: 0;
    left: 0; */
    /* use this ... */
    top: 120%;
    left: 0;
    width: 100%;
    height: 100%;

    /* this shifts the effective camera perspective as if 
    its looking down along 
    translateZ would move it up or down, but isn't needed here -/+ 
    scale reduces size */
    transform: perspective(1em) rotateX(40deg) scale(1, 0.35);
    /* makes it look more like a light is cast
    play with the value till you find a look you like */
    filter: blur(1.5em);
    /* opacity to adjust intensity, boost opacity to make it glow
    more on a hover */
    opacity: 0.7;
}

.neon-button::after {
    content: "";
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;

    box-shadow: 0 0 2em 0.5em var(--clr-neon);
    opacity: 0;

    transition: opacity 100ms linear;
}

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

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

.neon-button:is(:hover, :focus) {
    background-color: var(--clr-neon);
    color: var(--clr-bg);
    /* could also be unset */
    text-shadow: none;
}

/* below was from the original file, I commented it out and typed while watching the video*/

/* .neon-button {
  font-size: 4rem;
  display: inline-block;
  cursor: pointer;
  text-decoration: none;
  color: var(--clr-neon);
  border: var(--clr-neon) 0.125em solid;
  padding: 0.25em 1em;
  border-radius: 0.25em;
  text-shadow: 0 0 0.125em rgba(255, 255, 255, 0.3), 0 0 0.45em currentColor;
  box-shadow: inset 0 0 0.5em 0 var(--clr-neon), 0 0 0.5em 0 var(--clr-neon);
  position: relative;
} */

/* .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);
  opacity: 0;
  background-color: var(--clr-neon);
  z-index: -1;
  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;
} */