第一种方法： flex布局

布局：
<div class="square">
    <div class="square-inner">
        <div>1</div>
        <div>2</div>
        <div>3</div>
        <div>4</div>
        <div>5</div>
        <div>6</div>
        <div>7</div>
        <div>8</div>
        <div>9</div>
    </div>
</div>
样式：
.square{
    position: relative;
    width: 80%;
    padding-bottom: 80%;  /* padding百分比是相对父元素宽度计算的 */
}
.square-inner{
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%; /* 铺满父元素容器，这时候宽高就始终相等了  */
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
}
.square-inner>div{
    width: calc(100% / 3);
    height: calc(100% / 3);
    border: 5px solid #fff;
    background: red;
    box-sizing: border-box;
}

第二种： grid布局

布局：
<div class="square">
    <div class="square-inner">
        <div>1</div>
        <div>2</div>
        <div>3</div>
        <div>4</div>
        <div>5</div>
        <div>6</div>
        <div>7</div>
        <div>8</div>
        <div>9</div>
    </div>
</div>
样式：
.square{
    position: relative;
    width: 80%;
    padding-bottom: 80%;
}
.square-inner{
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    display: grid;
    grid-template-columns:repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    grid-gap: 2%;
    grid-auto-flow:column;
}
.square-inner>div {
    background: red;
}