clip-path (inset()/polygon()/circle()) hides everything outside the shape while the element still occupies its normal layout box. Transitioning between two shape values lets the browser interpolate the shape, producing a wipe-style reveal.
Smooth interpolation only works between polygon()s with the same number of points — a mismatched vertex count snaps instead of animating.
When to use
Use for scroll reveals, image gallery transitions, hero entrance animation. Older browsers lack support, so plan a fallback.