Sizzle.attr = function(elem, name) {
    // Set document vars if needed
    if ((elem.ownerDocument || elem) !== document) {
        setDocument(elem);
    }

    var fn = Expr.attrHandle[name.toLowerCase()],
        // Don't get fooled by Object.prototype properties (jQuery #13807)
        val = fn && hasOwn.call(Expr.attrHandle, name.toLowerCase()) ?
            fn(elem, name, !documentIsHTML) :
            undefined;

    return val !== undefined ?
        val :
        support.attributes || !documentIsHTML ?
        elem.getAttribute(name) :
        (val = elem.getAttributeNode(name)) && val.specified ?
        val.value :
        null;
};

// Support: IE<8
// Verify that getAttribute really returns attributes and not properties (excepting IE8 booleans)
support.attributes = assert(function(div) {
    div.className = "i";
    return !div.getAttribute("className");
});

getAttribute("") 方法返回属性的值。

getAttributeNode("") 方法返回属性节点，getAttributeNode('').value取得节点值。

<li class="aa"></li>

li.getAttributeNode('class')  // class="aa"
li.getAttribute('class') //aa