Plain HTML, CSS and JavaScript with no build step. Supports endless, target, auto-reset, cycle and countdown modes, adjustable targets and step size, keyboard controls, optional vibration and screen wake lock, and Light, Dark, System and pure-black AMOLED themes.
It works offline after the first load, keeps all state in local storage shared between tabs, and ships with tests for mode transitions, malformed storage, cross-tab coordination, themes and service-worker behaviour.