How to make an alert pop up with Javascript only when an innerHTML is over 99?

First timer and javascript newbie here. I'm learning javascript a bit late, and I've run into a problem.

I have a counter that goes up by one when a user clicks a button. The code is here:

<script language="javascript">  
var cnt = 0;  
function cntClicks(){  
cnt = cnt + 1;  
document.getElementById('test').innerHTML = cnt;  
}  
</script>  
<h1 id="test">0</h1>
<button onclick="cntClicks();">add 1</button>

So far it works, but I want an alert to pop up when someone clicks 100 times. I have no idea how to execute this, and also I would sincerely appreciate the answer included in HTML. This is because Wapka doesn't allow purely javascript files. Was I correct to think this would include an if statement focused on the innerHTML being > 99?