Class attribute in FAQ Stripped out

Hi Guys

Just trying to use a div with a class attribute in a FAQ but each time I save it the class is removed.

e.g.

<div class="embed-container"><iframe src='https://player.vimeo.com/video/1234567' frameborder='0' webkitAllowFullScreen mozallowfullscreen allowFullScreen></iframe></div>

When I add the above code (class="embed-container") is stripped out!

How can I make sure class="embed-container" is not removed from my FAQ article?