Javascript Fenêtre de confirmation Yes/No

<style>
#confrmdiv
{
    display: none;
    background-color: #eee;
    border-radius: 5px;
    border: 1px solid #aaa;
    position: fixed;
    width: 300px;
    left: 50%;
    margin-left: -150px;
    padding: 6px 8px 8px;
    box-sizing: border-box;
    text-align: center;
}
#confrmdiv button {
	background-color: #ccc;
	display: inline-block;
	border-radius: 3px;
	border: 1px solid #aaa;
	padding: 2px;
	text-align: center;
	width: 80px;
	cursor: pointer;
float:none;
}
#confrmdiv .button:hover
{
    background-color: #ddd;
}
</style>

<script>
 function dialogEnregistrerSiModifié(){
    document.getElementById('confrmdiv').style.display="block";
    document.getElementById('oui').onclick = function(){
        //alert('true');
        document.getElementById('confrmdiv').style.display="none";
    };
    document.getElementById('non').onclick = function(){
        document.getElementById('confrmdiv').style.display="none";
        return false;
    };
 }
</script>

    <button onclick="dialogEnregistrerSiModifié()">confirmation</button>

    <div id="confrmdiv">Vous confirmez?<br/>
        <button id="oui">Yes</button>
        <button id="non">No</button>
    </div>