## TL;DR

The Fullscreen API enables web content to request presentation in full-screen mode (`element.requestFullscreen()`), hiding browser UI. User gesture is required, and `document.exitFullscreen()` restores normal view.

## Core Explanation

`document.fullscreenElement` checks current fullscreen element. CSS pseudo-classes: `:fullscreen` styles the fullscreen element, `::backdrop` styles the background. Keyboard access is restricted in fullscreen (no Esc to exit for all keys — only Esc exits by spec). The API is available only in secure contexts.

## Further Reading

- [Fullscreen API Standard](https://fullscreen.spec.whatwg.org/)