JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div class="main"></div>
    <div class="span2">
        <div><a href="xyz71">List 1</a></div>
        <div>
            <ul>
                <li><a href="xyz71_72">Cat 11</a>
                </li>
                <li><a href="xyz71_73">Cat 12</a>
                </li>
                <li><a href="xyz71_76">Cat 13</a>
                </li>
                <li><a href="xyz71_78">Cat 14</a>
                </li>
            </ul>
        </div>
    </div>
    <div class="span2">
        <div><a href="xyz90">List 2</a></div>
        <div>
            <ul>
                <li><a href="xyz90_91">Cat 211</a>
                </li>
                <li><a href="xyz90_92">Cat 212</a>
                </li>
                <li><a href="xyz90_93">Cat 213</a>
                </li>
            </ul>
        </div>
        <div>
            <ul>
                <li><a href="xyz90_91">Cat 221</a>
                </li>
                <li><a href="xyz90_92">Cat 222</a>
                </li>
                <li><a href="xyz90_93">Cat 223</a>
                </li>
                <li><a href="xyz90_94">Cat 224</a>
                </li>
                <li><a href="xyz90_95">Cat 225</a>
                </li>
                <li><a href="xyz90_96">Cat 226</a>
                </li>
            </ul>
        </div>
    </div>
</div>

CSS

#container > div {
    position: relative;
}
#container > div div:nth-child(3):after {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    border: 1px solid #000;
    z-index: -1;
}