Flex layout broken in IE11

Question

I'm completely lost, I read so much things and I still couldn't figure it out how to fix this layout working in IE11 :(

HTML

<div class="site">
  <header>
    <div class="wrap">
      Header
    </div>
  </header>
  <main class="site-content">
    <div class="wrap">
      <aside>
        <ul>
          <li>Menu 1</li>
          <li>Menu 2</li>
        </ul>
      </aside>
      <article>
        <h1>Hello</h1>
      </article>
    </div>
  </main>
  <footer>
    <div class="wrap">Footer</div>
  </footer>
</div>

CSS

body, html {
  margin: 0;
}

.site {
  display: flex;
  min-height: 100vh;
  flex-direction: column;
}

.site-content {
  flex: 1;
  display: flex;
}

aside {
  float: left;
  width: 25%;
  background: #ccc;
  margin-right: 5%;
}

article {
  float: left;
  width: 70%;
}

footer {
  background: #eee;
}

header {
  background: tomato;
}

.wrap {
  max-width: 960px;
  margin: 0 auto;
  flex: 1;
  display: flex;
}

DEMO: http://codepen.io/anon/pen/WoZJWj

It's working in every browsers but IE11 makes me crazy. Thanks in advance