<html>
<head>
<style>
.bar{background: red}
</style>
</head>
<body>
<div class="foo">foo</div>
<div class="foo">foo</div>
<div class="foo">foo</div>
<div class="foo">foo</div>
<div class="foo">foo</div>
<div class="foo">foo</div>
<div class="foo">foo</div>
<script>
getAll=document.querySelectorAll.bind(document)
c=0
;[].forEach.call(getAll(".foo"), function(el){if(((c++)%2!=0)) el.className+=" bar"})
</script>
</body>
</html>