Depth is just
speed differences.

Every layer scrolls at its own speed.

The scene you just scrolled through is five transparent images stacked on top of each other, and a CSS gradient for the sky. As the page moves, each layer is nudged back down by a different fraction of the scroll distance — the far ridge barely moves, the treeline in front moves at full speed. Your brain reads the speed difference as depth.

No plugin. No video. No library. About 20 lines of JavaScript.

Scroll back up and watch the tree tips slide across the ridgeline behind them — that slide is the whole trick.