Engine lab
Test fallback behavior in Safari or Firefox, or enable Force fallback in Chrome to exercise the JS engine.
Hover
Instant update vs transition on border-width (run + end
only, not mid-transition).
Nested scroll
IntersectionObserver defers off-screen items inside
overflow: auto.
Size animation
Width / height keyframes animate via ResizeObserver.
Layer mode
Box-shadow and outline use SVG pseudo-layers.