Just a moment...
codepen.io

Single HTML element + CSS only

  1. Inhale for 4 seconds
  2. Hold for 4 seconds
  3. Exhale for 4 seconds
  4. Hold for 4 seconds

And repeat

Inspired by: https://quietkit.com/box-breathing/

Note: The current Safari version has a bugged linear() implementation that has been fixed in the upcoming version.