How can we center an iframe inside an absolute positionned div

How can we center an iframe inside an absolute positionned div

I tried to put the iframe tag inside an a tag and set its css to
text-align:center, but it didn't work
Edit Sorry, I mislead I have an iframe
<div>
<iframe type="text/html" width="640" height="360"
src="https://www.youtube.com/..." frameborder="0"
allowfullscreen=""></iframe>
<button type="button">Ajouter la vidéo...</button>
</div>
the div is itself centered and has class
width: 500px;
position: absolute;
top: 35%;
left: 30%;
margin-left: -200px;
z-index: 3;