CSS targeting specific images

Question

I am looking for a way to target a specific image with CSS, if this is even possible.

I am running a CMS where the users can upload images and put them on their page

Answer 1

img[width="400"]{
   float:left;
}

This will make any image that have the attribute set to 400 float left:

Note: this will only work if the width attribute is set. Will not work if the image is 400px wide but does not have a width attribute set.