<picture alt="description of image">
<source src="small.jpg">
<source src="medium.jpg" media="(min-width: 400px)">
<source src="large.jpg" media="(min-width: 800px)">
<img src="small.jpg" alt="description of image">
</picture>
li {
float:left;
}
li:nth-child(3n+1) {
clear:both;
}