A Countdown Timer with reset functionality in pure Javascript
From a short review;

You use 9 twice, this should be assigned to a nicely named global constant
You hard code both 9 and 00:10, this is not flexible. I would expect to be able to change 9 to 99 and to have the timer start at 01:40
Similarly, 00:0${startingTime--} does not allow for much flexibility in the script
The browser tries to honor the 1000, but it cannot be trusted. You are supposed to keep track of the original start and deduct that from current time to determine how much time has passed. Otherwise the timer is not reliable.
Placing the timer in a h1 element is probably not a good idea semantically
Your entire code should be in a well named (self executing) function
I am not sure why you return anything in startStopTimer? If you are never using the returned value, then just drop the return keyword
since clearInterval returns undefined and because it makes sense, I would go for startTimer = clearInterval(startTimer);
Why console.clear()? Production code should avoid console