Press the fullscreen button or the F key and the countdown expands to fill the entire screen — huge, high-contrast digits with everything else stepped out of the way. It's built for situations where the timer needs to be readable at a glance from a distance: a classroom, a conference room, a stage, a gym floor.

Fullscreen mode deliberately keeps only what you need mid-session:

• The countdown itself, at its largest size.
• Start / Pause / Reset, always reachable.
• A mute toggle, so alarm status is visible without opening a settings panel.
• An explicit exit-fullscreen control (or just press Escape).

Everything else — presets, the duration inputs, alarm sound selection, the site navigation — is hidden until you exit.

On devices and browsers that support it, entering fullscreen also requests a screen wake lock, so the display won't automatically dim or sleep partway through a presentation or workout. If your browser doesn't support the Fullscreen API at all (this is rare, but happens on some mobile browsers), the timer falls back to a large, full-viewport display mode automatically — the experience degrades gracefully rather than breaking.