<!DOCTYPE HTML>
<html>
    <head>
        <meta charset="utf-8" lang="en-us"/>
        <title>gCube demo</title>
        <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
        <script src="google-cube-code.js"></script>
        <script src="gcube.js"></script>
        <style>
            .centerContent {
                text-align: center;
            }
        </style>
    </head>
    <body class="centerContent">
        <div id="main" style="margin-left:auto;margin-right:auto;">
            <h1 style="text-align: center">gCube Demo</h1>
            <h2 style="text-align: center">Sune: R U R' U R U2 R'</h2>
            <g-cube>
                <g-scramble>R U2 R' U' R U' R'</g-scramble>
                <g-algorithm>R U R' U R U2 R'</g-algorithm>
                <g-highlight>OLL</g-highlight> 
                <g-florian>0.3em,0.3em</g-florian>
            </g-cube>
        </div>
    </body>
</html>

<!DOCTYPE HTML>
<html>
    <head>
        <meta charset="utf-8" lang="en-us"/>
        <title>gCube demo</title>
        <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
        <script src="google-cube-code.js"></script>
        <script src="gcube.js"></script>
        <style>
            .centerContent {
                text-align: center;
            }
        </style>
    </head>
    <body>
        <div id="main" class="centerContent" style="margin-left:auto;margin-right:auto;">
            <h1 style="text-align: center">gCube Demo</h1>
            <h2 style="text-align: center">Sune: R U R' U R U2 R'</h2>
            <g-cube>
                <g-scramble>R U2 R' U' R U' R'</g-scramble>
                <g-algorithm>R U R' U R U2 R'</g-algorithm>
                <g-highlight>OLL</g-highlight> 
                <g-florian>0.3em,0.3em</g-florian>
            </g-cube>
        </div>
    </body>
</html>

Put the center class on the div instead of the body.
Put the center class on the div instead of the body.
