SOURCE

<div>
    <ul>
        <li>1</li>
        <li>2</li>
        <li>3</li>
        <li>4</li>
        <li>5</li>
        <li>6</li>
        <li>7</li>
    </ul>
</div>

*{
    list-style: none;
    margin: 0;
    padding: 0;
    box-sizing: border-box
}

div{
    resize: both;
    overflow: hidden;
}
ul{
    width: 100%;
    height: 100%;
    display: flex;
    flex-wrap: wrap;
    align-content: space-between;
    justify-content: space-between;
    background: black;
    margin-left: 10px; /* 能够让子元素顶边的关键属性 */
}
ul li{
    width: 50px;
    height: 50px;
    background: red;
    margin-right: 10px;
    flex-basis: 50px;
}