No it doesn’t: http://www.w3.org/TR/css3-flexbox/. To whit:

  <div id="container">
    <div id="centered"></div>
  </div>
  
  #container {
    display: flex;
    width: 1000px;
    height: 1000px;
    background: gray;
  }
  #centered {
    margin: auto;
    width: 500px;
    height: 500px;
    background: red;
  }

Of course, we’re waiting on implementations to catch up with the spec: http://caniuse.com/#feat=flexbox . That’s not really the W3’s fault though.

That "spec" was a Working Draft in June 2012 and a Candidate Recommendation in September 2012. It's still not final. And browser vendors have implemented prefixed versions (even Microsoft!). I think it is reasonable to criticise the W3 for not finalising something we've been struggling with for over fifteen years.

Does that also work when you don't know the width and height of container and centered in advance?