I am trying to center some divs vertically and horizontally using flexbox but it's not working as expected.
here is an image illustrating what am having and what I expect.

And here is the code:
<html lang="en">
<body>
<style type="text/css">
body{
display: flex;
flex-direction: column;
justify-content: center;
align-items: center;
}
</style>
<div>111111</div>
<div>222222</div>
<div>333333</div>
</body>
</html>
EDIT
Of course, I know it'd be better puting the divs in a container and that's what I tried first but in that case, I end up with something like this:

So as you can see, the divs are get centred in the container as expected but the container itself is not.
Your code is fine, you should only add a min-height: 100vh to the body so the div can be centered in the viewport height (the body doesn't always cover the viewport height, it just takes minimal space to contain its children)
Example: http://codepen.io/anon/pen/xGQpKO
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