[monolithed]

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window.onload = function(){
    var li = document.getElementById('ul').getElementsByTagName('li'), i = li.length;
    while(i--) {
        li[i].style.backgroundColor = i%2 ? 'red' : 'green';
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<ul id="ul">
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
</ul>