with dom if

<!DOCTYPE html>
<html style="height: 100%">
<head>
    <meta charset="utf-8">
    <title>JS Bin</title>
    <link rel="import" href="https://raw.githubusercontent.com/Polymer/polymer/master/polymer.html">
    <dom-module id="my-element">

        <template>
            <template is="dom-if" if="true">
            <div style="width: 100%; height: 100%; min-height: 100%; display: flex; justify-content: center; align-items: center; ">
                <content></content>
            </div>
            </template>
        </template>

        <script>
            Polymer({
                is: "my-element"
            });
        </script>

    </dom-module>

</head>
<body style="width: 100%; height: 100%; min-height: 100%; ">

<my-element>
    <div>xxx</div>
</my-element>


</body>
</html>

// Code goes here

/* Styles go here */