// 邮箱输入框失去焦点时，根据输入框中是否内容设置label的样式
email_input.onblur = function() {
    var form_label_content = email_input.nextElementSibling.children[0];
    if (email_input.value) {   
        form_label_content.style.transform = 'translateY(-15px)';
        form_label_content.style.fontSize = '0.9em';
    } else {
        // reset
    }
};

<div class="container">
    <div class="cube-box"> <!-- 加一层 -->
        <div class="cubic">
            <div class="front">font</div>
            <div class="back">back</div>
            <div class="left">left</div>
            <div class="right">right</div>
            <div class="top">top</div>
            <div class="bottom">bottom</div>
        </div>
    </div>
</div>

.container .cube-box {
    /* 景深效果 */
    perspective: 1000px;
    -moz-perspective: 1000px;
    -ms-perspective: 1000px;
    -webkit-perspective: 1000px;
    -moz-perspective: 1000px;

    /* 更改下透视点的位置 */
    perspective-origin: 50% 50%;
}

.container .cubic {
    width: 500px;
    height: 500px;
    position: relative;
    display: flex;
    /* 使cubic中的6个div水平、垂直居中*/
    justify-content: center;
    align-items: center;

    /* 子元素保持3D效果 */
    transform-style: preserve-3d;
    -webkit-transform-style: preserve-3d;
    -moz-transform-style: preserve-3d;
    -o-transform-style: preserve-3d;
    -ms-transform-style: preserve-3d;
}