CSS Flexbox vertical centering

This is horizontally and vertically centered with flexbox.

HTML

<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Vertically Centered with CSS Flexbox</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<body>
<main>
<h3>Welcome to CSS Flexbox</h3>
<p>This is horizontally and vertically centered.</p>
</main>
</body>
</html>

CSS

body, html {
margin: 0;
padding: 0;
height: 100%;
display: flex;
align-items: center;
justify-content: center;
font-family: Calibri, Candara, Segoe, Segoe UI, Optima, Arial, sans-serif;

/** font size grows 1px for every 100px of viewport width **/
font-size: calc(16px + 1vw);

/** line-height grows with font size **/
line-height: calc(1.1em + 0.5vw);
background: url(https://placeimg.com/1200/1200/nature) center center fixed;
background-size: cover;
}

main {
background: rgba(0, 0, 0, 0.70);
width: 75%;
color: #EAEAEA;
padding: 10%;
text-align:center;

}