CSS를 사용하여 9개의 정사각형 그리드 레이아웃을 구현하는 여러 가지 방법에 대해 이야기해 보겠습니다!

<div class="box">
    <ul>
        <li>1</li>
        <li>2</li>
        <li>3</li>
        <li>4</li>
        <li>5</li>
        <li>6</li>
        <li>7</li>
        <li>8</li>
        <li>9</li>
    </ul>
</div>

.box {
    background: #e4f7fd61;
    border: 2px solid #0786ada1;
    border-radius: 8px;
}

ul {
    padding: 0; 
    margin: 0;
}

.box li {
    list-style: none;
    text-align: center;
    line-height: 200px;
    background: skyblue;
    border-radius: 8px;
    font-size: 20px;
    color: black;
}

.box {
    width: 100%;
    overflow: hidden;
}

ul {
    width: 100%;
    height: 100%;
    display: flex;
    flex-wrap: wrap;
}

.box li {
    width: 30%;
    height: 30%;
    margin-right: 5%;
    margin-bottom: 5%;
}

.box li:nth-of-type(3n) {
    margin-right: 0;
}

.box li:nth-of-type(n+7) {
    margin-bottom: 0;
}

.box {
    width: 100%;
    overflow: hidden;
}

ul {
    width: 100%;
    height: 100%;
}

.box li {
    width: 30%;
    height: 30%;
    margin-right: 5%;
    margin-bottom: 5%;
    float: left;
}

.box li:nth-of-type(3n) {
    margin-right: 0;
}

.box li:nth-of-type(n+7) {
    margin-bottom: 0;
}

<div class="grid">
    <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>

box {
    background: #e4f7fd61;
    border: 2px solid #0786ada1;
    border-radius: 8px;
}

.grid {
    display: grid;
    width: 100%;
    grid-template-rows: repeat(3, 1fr);
    grid-template-columns: repeat(3, 1fr);
    grid-gap: 5%;
    grid-auto-flow: row;
}

.grid>div {
    list-style: none;
    text-align: center;
    line-height: 200px;
    background: skyblue;
    border-radius: 8px;
    font-size: 20px;
    color: black;
}

<div class="box">
    <ul>
        <li>
            <div>1</div>
            <div>2</div>
            <div>3</div>
        </li>
        <li>
            <div>4</div>
            <div>5</div>
            <div>6</div>
        </li>
        <li>
            <div>7</div>
            <div>8</div>
            <div>9</div>
        </li>
    </ul>
</div>

.box {
    width: 100%;
    overflow: hidden;
}

ul {
    width: 100%;
    height: 100%;
    display: table;
    border-spacing: 10px;
}

.box li {
    display: table-row;
}

.box li div {
    display: table-cell;
    text-align: center;
    border-radius: 10px;
}