This example showcases a minimal HTML page where the text “Hello HTML” slides down from the top of the viewport using a CSS animation. The page defines a `@keyframes` rule named `slidein`, which animates the body’s `margin-top` from 100% to 0% over three seconds. The animation is applied directly to the body element via the `animation-name` and `animation-duration` properties. This demonstrates a purely CSS-driven approach without JavaScript or external libraries, relying solely on the browser’s native animation engine.