This question has been answered by iusemodx. See the first response.
:nth-child(odd){} :nth-child(even){} li:nth-child(odd){} li:nth-child(even){} .class:nth-child(odd){} .class:nth-child(even){}
.bloglist section:nth-child(even) img { float:right;} .bloglist section:nth-child(odd) img { float:left;} .bloglist img { display: block; padding: 5px; background-color: #FFFFFF; border: 1px solid #56486F; margin: 7px;}