css nth-child的用法：选择器匹配属于其父元素的第 N 个子元素

html代码

<div class="test-container">
    <div class="test-card">
        <img src="images/test.jpg" alt="">
    </div>
    <div class="test-card">
        <img src="images/test.jpg" alt="">
    </div>
    <div class="test-card">
        <img src="images/test.jpg" alt="">
    </div>
    <div class="test-card">
        <img src="images/test.jpg" alt="">
    </div>
    <div class="test-card">
        <img src="images/test.jpg" alt="">
    </div>
    <div class="test-card">
        <img src="images/test.jpg" alt="">
    </div>
</div>

css代码

.test-container {
    display: flex;
    flex-wrap: wrap;
    width: 90%;
    margin: 0 auto;
}
.test-card{
    width: 23.5%;
    position: relative;
    padding-bottom: 13.22%;
    margin-bottom: 35px;
}
.test-card:nth-child(4n-2){
    margin: 0 2% 35px;
}
.test-card:nth-child(4n-1){
    margin-right: 2%;
}

.test-card img{
    width: 100%;
    height: 100%;
    position: absolute;
    top: 0;
    left: 0;
}

最终效果