JSFiddle - React, Tailwind, and code Playground

HTML

<h1>Countdown Timer</h1>
   <div id="mainCont">
   <input type="text" id="timeEntered">
   <p>
     <button id="startPause" onclick="startPause()">Start/Stop</button>
   </p>
     <div id="output">00:00:00</div>
  </div>

JavaScript

var running = 0; //Glob variable for starting/pausing timer

   function startPause(){ 
      var time = document.getElementById("timeEntered").value; //Not sure if needed but I just have the time entered be converted to seconds.
      var a = time.split(":");
      var timeToSeconds = (+a[0]) * 60 + (+a[1]) * 60 + (+a[2]);
      if(running == 0){ //If off, turn it on. 
         running = 1;
         countDown();
         document.getElementById("startPause").innerHTML = "Start/Stop";
      }else{
        running = 0;
        document.getElementById("startPause").innerHTML = "Resume";
      }
   }

   function countDown(timeToSeconds) {
      var time = document.getElementById("timeEntered").value; //Take user input and convert 00:00:00 format to seconds. 
      var a = time.split(":");
      if(!timeToSeconds)
      	var timeToSeconds = (+a[0]) * 60 + (+a[1]) + (Math.floor(+a[2]/100));
      if(running == 1){ //When user clicks start it will calculate the minutes, seconds, and milliseconds. 
         var minutes = Math.floor(timeToSeconds / 60) % 60;
         var seconds = Math.floor(timeToSeconds) % 60;
         var milli = Math.floor(timeToSeconds*100) % 100;
               console.log(milli);
         if(minutes <= 9) { //Add leading zeroes to display countdown in 00:00:00 format.
             minutes = "0" + minutes;
         }
         if(seconds <= 9) {
             seconds = "0" + seconds;
         }
         if(milli <= 9) {
             milli = "0" + milli;
         }
         timeToSeconds-=0.1; //Decrement the time entered. 
         //console.log(minutes + ":" + seconds + ":" + milli);
         document.getElementById("output").innerHTML = minutes + ":" + seconds + ":" + milli //Display the time 00:00:00 format. 
         if(timeToSeconds <= 0){ //When time is 00:00:00 the message will show. 
             document.getElementById("output").innerHTML = "The time is over."
             return;
         }
         else if(timeToSeconds !== -1){
         ...