问CSS3中的逆向3d旋转-透视-原点，变换-原点？

EN

<div class="grandparent">
<div class="parent">
    <div class="child1"></div>
    <div class="child2"></div>
</div>
</div>

.grandparent{
    -webkit-perspective: 500;
    -webkit-transform-style: preserve-3d;
    -webkit-perspective-origin: 50% 50%;
}

.parent {
    position: relative;
    background-color: yellow;
    width: 200px;
    height: 150px;
    -webkit-transform: rotateY(30deg) ;
}

.child1 {
    position: absolute;
    top: 20px;
    left: 20px;
    background-color: green;
    width: 50px;
    height: 50px;
    -webkit-transform: rotateY(-30deg) ;
}

.child2 {
    position: absolute;
    top: 20px;
    left: 120px;
    background-color: green;
    width: 50px;
    height: 50px;
    -webkit-transform: rotateY(-30deg) ;
}