<!DOCTYPE html>

<html>

<head>

    <meta charset="UTF-8">

    <title>隔行变色</title>

    <script>

        window.onload = function () {

            var aLi = document.getElementsByTagName('li');

            for (var i = 0; i < aLi.length; i++) {

                if (i % 2 == 0) {

                    aLi[i].style.background = '#333';

                }else{

                    aLi[i].style.background='red';

                }

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<body>

<ul>

    <li>2</li>

    <li>2</li>

    <li>2</li>

    <li>2</li>

    <li>2</li>

    <li>1</li>

</ul>

</body>

</html>