html 
    ----------
    <div id="countdown">
    <span> 10 </span>Minutes
    <span> 10 </span>Seconds <br>
    <inputtype="button"value="Start">
    </div>

    css
    ----------
    #countdown{width: 300px;text-align: center;background:;margin:30px auto;border:2px dashed blue;padding: 10px;}
    input{width: 100px;height: 50px;cursor: pointer; margin: 10px;}
    span{color:#000;width:80px;background:#fbfbfb;border:2px solid #b4b4b4;margin:10px;padding:0 10px;}
    input.cancel{background-position:0 -50px;}

    js
    ----------
    var oCountDown = document.getElementById("countdown");
    var aInput = oCountDown.getElementsByTagName("input")[0];
    var timer = null;
    
    aInput.onclick = function ()
    {
    //console.log(this.className);
    if(this.className==='')
    {
    timer=setInterval(updateTime,1000);
    }
    else{clearInterval(timer);}
    
    // this.className == "" ? (timer = setInterval(updateTime, 1000), updateTime()) : (clearInterval(timer));
    
    // to ensure the button works both for start and stop and also have cancel className
    if (this.className==='')
    {this.className='cancel'}else{this.className=''}
    
    //this.className = this.className === '' ? this.className="cancel" : '';
    };
    
    function updateTime()
    {
        var aSpan = oCountDown.getElementsByTagName("span");
        //oRemain is the total seconds
        var oRemain = aSpan[0].innerHTML * 60 + parseInt(aSpan[1].innerHTML);
        // The parseInt function converts its first argument to a string, parses it, and returns an integer or NaN.
        
        // close the timer,so it stops at 00 Minutes 00 seconds
        if (oRemain <= 0) 
        { 
            clearInterval(timer); return 
        }
        else
        {
            oRemain--;
        } 
        
        aSpan[0].innerHTML=format(parseInt(oRemain/60));
        oRemain %=60; 
        aSpan[1].innerHTML=format(parseInt(oRemain)); } 
        
        function format(a) 
        { 
            //The toString() method converts a number to a string. 
            return a.toString(); 
        }