Prompt

Make a focus timer that counts down 25 minutes. It has to be a single HTML file that opens in a browser with nothing to install and no other files. Put the CSS and JavaScript inside it.

Show the remaining time large, in the middle of the screen. Three buttons are enough: start, pause, and reset. When the time is up, the whole screen changes colour and large text appears.

Pausing and starting again should continue from where it stopped, and reset should go back to the time I set. If I look at another tab for a while, the count must not drift, so work out the remaining time from the clock rather than counting ticks.

Do not use image files or any external address. If you need a sound, use a short one the browser can generate itself.

When you are done, go through start, pause, start, reset as if you were pressing them. Fix anything that does not line up and give me only the final version.