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;}