The app is not finished yet. I will add js code to it…
Update (1/1/2021): The app is working Okay now. I add two beep sounds at the end of the timer. Also the NoSleep js is added to prevent screen sleeping. The only major problem is that if you refresh the webpage, the timer will go back to initial state 15 minutes.
Update (1/5/2021): Change the sound to three beeps and change the font color to red when it reaches zero. So far I am happy with the app.