SOURCE

    function nextNode(node, rootNode) {
        if (node.children.length) {
            return node.children[0]
        }
        if (node === rootNode) {
            return null
        }
        while (node.parentNode) {
            if (node.parentNode.nextElementSibling) {
                return node.parentNode.nextElementSibling
            }
            node = node.parentNode
        }
        return null
    }

    function nextSelectorNode(node, selector, rootNode) {
        return {
            'child': {value: nextNode(node, rootNode), hasNext: false},
            'descendant': {value: nextNode(node, rootNode), hasNext: true},
            'subSelector': {value: nextNode(node, rootNode), hasNext: true},
        }[selector.relation]
    }

    function match(node, selector) {
        return {
            'class': node.className.trim().split(/ +/).indexOf(selector.value) >= 0,
            'tag': node.tagName.toLowerCase() === selector.value.toLowerCase(),
        }[selector.matchType]
    }

    function querySelector(node, selectors) {
        var rootNode = node
        node = {value: node, hasNext: true}
        let i = 0
        for (i; i < selectors.length; i++) {
            node = nextSelectorNode(node.value, selectors[i], rootNode)
            do {
                if (match(node.value, selectors[i])) {
                    break
                }
                node = nextSelectorNode(node.value, selectors[i], rootNode)
            } while (node.value && node.hasNext)
        }
        if (i === selectors.length) {
            return node
        }
        return null
    }

    //把selector解析为
    var selectors = [
        {relation: 'subSelector', matchType: 'class', value: 'mls-info'},
        {relation: 'child', matchType: 'tag', value: 'div'},
        {relation: 'descendant', matchType: 'class', value: 'copyright-content'},
    ]

    console.log(querySelector(document, selectors))

<div class="mls-info">
    <div>
        <div class="copyright-content"></div>
    </div>
</div>