如何将元素堆叠在彼此之上？

<style>
    .parent {
        position: relative;
    }
    .child {
        position: absolute;
        top: 0;
        left: 0;
        z-index: 1;
    }
    .child2 {
        position: absolute;
        top: 0;
        left: 0;
        z-index: 2;
    }
</style>

<div class="parent">
    <div class="child">子元素1</div>
    <div class="child2">子元素2</div>
</div>

<style>
    .parent {
        position: relative;
        z-index: 1;
    }
    .child {
        position: relative;
        z-index: 2;
    }
    .child2 {
        position: relative;
        z-index: 3;
    }
</style>

<div class="parent">
    <div class="child">子元素1</div>
    <div class="child2">子元素2</div>
</div>