<ul class="domtree">
        <li>
            1级菜单
            <ul>
                <li>2级菜单</li>
                <li>
                    2级菜单
                    <ul>
                        <li>3级菜单</li>
                        <li>3级菜单</li>
                    </ul>
                </li>
            </ul>
        </li>
        <li>
            1级菜单
            <ul>
                <li>2级菜单</li>
                <li>2级菜单</li>
            </ul>
        </li>
    </ul>

        ul.domtree,
        ul.domtree ul {
            margin: 0;
            padding: 0 0 0 2em;
        }

        ul.domtree li {
            list-style: none;
            position: relative;
        }

        ul.domtree>li:first-child:before {
            border-style: none none solid none;
        }

        ul.domtree li:before {
            position: absolute;
            content: '';
            top: -0.01em;
            left: -0.7em;
            width: 0.5em;
            height: 0.615em;
            border-style: none none solid solid;
            border-width: 0.05em;
            border-color: #aaa;
        }

        ul.domtree li:not(:last-child):after {
            position: absolute;
            content: '';
            top: 0.7em;
            left: -0.7em;
            bottom: 0;
            border-style: none none none solid;
            border-width: 0.05em;
            border-color: #aaa;
        }