Hello here is a project i will suggest beginner learners to try.

My solution

Calling my function watchtime() with a setInterval of 9 ( Why 9 Instead of 10 ? )

In my case it didn't showed exact interval using 10.
function watchtime() {
    mili.innerHTML = (countMili > 9) ? countMili : "0" + countMili;
    if (countMili == 99)
        countSec++;
    sec.innerHTML = (countSec > 9) ? countSec : "0" + countSec;
    countMili = (countMili < 99) ? countMili + 1 : 0;
    countSec = (countSec < 60) ? countSec : 0;
    if (countSec == 0 && countMili == 0) {
        countMin++;
        min.innerHTML = (countMin > 9) ? countMin : "0" + countMin;
    }
}

In this project i have tried to add an additional button called "LAP".

What will it do?

When the LAP time button is pressed while the watch is running it allows the elapsed time to that point to be read.

Hope this will help <3