/* It can sometimes be desirable to remove the top or left margin from the first element in a container. Likewise, the right or bottom margin from the last element in a container. You can do this by manually applying classes to the HTML: */
.first { margin-top: 0 !important; margin-left: 0 !important; }
.last { margin-bottom: 0 !important; margin-right: 0 !important; }
/* The "top"/"bottom" zeroing being useful with a vertical stack of elements, "left"/"right" zeroing being useful for horizontal rows (in general). But... this method is dependent on you adding classes to the HTML yourself. Pseudo-selectors can be a better less intrusive way to go: */
* > :first-child { margin-top: 0 !important; margin-left: 0 !important; }
* > :last-child { margin-bottom: 0 !important; margin-right: 0 !important; }
/* You may want to replace the * with more specific selectors as per your needs.
"Every Third", etc.
Lets say you had a floated block of 9 elements, 3 by 3. It's very common that you might need to remove the right margin from the 3rd, 6th, and 9th items. The nth-child pseudo-selector might be able to help there: */
* > :nth-child(3n+3) { margin-right: 0; }
/* The equation there, 3n+3, works like this:
(3x0)+3 = 3
(3x1)+3 = 6
(3x2)+3 = 9
etc.
jQuery
jQuery uses CSS3 selectors, which includes :first-child, :last-child, and :nth-child(). This means that in browsers with don't or don't fully support these selectors, they WILL work in jQuery, so you can substitute the CSS support with JavaScript support. For example: */
$("* > :nth-child(3n+3)").css("margin-right", 0);
/* Browser support
:first-child and :last-child works in the latest release from all major browsers, but not in IE 6. :first-child is supported in IE 7+. :nth-child works in Safari 3+, Firefox 3.5+, and Chrome 1+, but still doesn't work in IE8. */