Thanks for posting a screenshot. After looking through your code, here are some things to look into:
(1) does IE 7 support multiple class names per class? You have up to three. For example: <div class="box half row">1/2</div>
(2) does IE 7 support .className + elementName selector? I know IE 7 supports elementName + elementName as long as you have a doctype.
(3) does IE 7 support long decimal numbers? You have 74.5000% for example.
(4) IE 7 might require you to clear: both, right after your <div class="box onefourth row"> div element.
Check out this article for info about what selectors IE7 supports: