通过CSS使用：before和：after伪元素来模拟方括号.

.matrix {
        position: relative;
    }
    .matrix:before,.matrix:after {
        content: "";
        position: absolute;
        top: 0;
        border: 1px solid #000;
        width: 6px;
        height: 100%;
    }
    .matrix:before {
        left: -6px;
        border-right: 0;
    }
    .matrix:after {
        right: -6px;
        border-left: 0;
    }

<div align=center>
    <table class="matrix">
        <tr>
            <td>1+3i</td>
            <td>2+i</td>
            <td>10</td>
        </tr>
        <tr>
            <td>4-3i</td>
            <td>5</td>
            <td>-2</td>
        </tr>
    </table>
</div>