Who needs paper and a pen anymore? Just enter the amount to reduce or add using the simple buttons and (gently) hit the green confirm button!
By default each player starts a duel with 8000 LP, but if you're speed duelling you can change the starting LP to 4000 or 2000.
It's responsive to different screen sizes, works offline and is installable on your mobile device if you use Mobile Chrome/Android Webview v31+, Firefox for Android v58+ or Opera for Android v32+ as well as on your desktop if use Chrome OS (Chrome v67+), Linux (Chrome v70+) or Windows (Chrome v70+). Mac support for Desktop PWA supposedly expected to arrive in Chrome v72.
Use it on a device for each player or for both players, simply press the add player button. Utilise the rotation feature and place your device between you and your opponent.