<h1 split-by="letter" letter-animation="breath">
  animated letters
</h1>

        @keyframes breath {
  from {
    animation-timing-function: ease-out;
  }

  to {
    transform: scale(1.25) translateY(-5px) perspective(1px);
    text-shadow: 0 0 40px var(--glow-color);
    animation-timing-function: ease-in-out;
  }
}

@media (prefers-reduced-motion:no-preference) {
  [letter-animation] > span {
    display: inline-block;
    white-space: break-spaces;
  }

  [letter-animation=breath] {
    --glow-color: white;
  }

  [letter-animation=breath]>span {
    animation: breath 1.2s ease calc(var(--index) * 100 * 1ms) infinite alternate;
  }
}

@media (prefers-reduced-motion:no-preference) and (prefers-color-scheme: light) {
  [letter-animation=breath] {
    --glow-color: black;
  }
}
        

        const span = (text, index) => {
  const node = document.createElement('span')

  node.textContent = text
  node.style.setProperty('--index', index)
  
  return node
}

const byLetter = text =>
  [...text].map(span)

const {matches:motionOK} = window.matchMedia(
  '(prefers-reduced-motion: no-preference)'
)

if (motionOK) {
  const splitTargets = document.querySelectorAll('[split-by]')

  splitTargets.forEach(node => {
    let nodes = byLetter(node.innerText)

    if (nodes)
      node.firstChild.replaceWith(...nodes)
  })
}