Something like this should do the job
ul.zebra li:nth-child(odd), ol.zebra li:nth-child(odd) { font-weight: bold; }
And your markup will be
<ul class="zebra"> <li>List item</li> <li>List item</li> <li>List item</li> <li>List item</li> </ul>
or
<ol class="zebra"> <li>List item</li> <li>List item</li> <li>List item</li> <li>List item</li> </ol>
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