CSS Sticky Parallax Sections Demo

Nice demo on CodePen by Ryan Mulligan, featuring some sections with a sticky parallax background image:

See the Pen CSS Sticky Parallax Sections by Ryan Mulligan (@hexagoncircle) on CodePen.

I expected to find the the translateZ() + scale() method to create the parallax layers in there, but turns out Ryan took another approach:

1. Scale down each enclosing section, using a scaleY transform with a number lower than 1, e.g. scaleY(0.9)
2. Scale up each contained direct child of those sections using an inverse scaleY transform, e.g. scaleY(1.11111111)

As the second scaling (1.11111) undoes the first scaling (0.9), the content is rendered at the original scale of 1 again (e.g. 0.9 * 1.11111 = 1).

:root {
  --speed: 0.1;
}

.section {
  transform-origin: center top;
  transform: scaleY(calc(1 - var(--speed)));
}

.section > * {
  transform-origin: center top;
  transform: scaleY(calc(1 / (1 - var(--speed))));
}