I have this fragment class which marks an area with red. My system is separated into two types: categories and rules. When rules are fragmented, they receive the new css class well and it looks properly, but it’s different with categories, they have that unnecessary white-space in top of the category.
Here is a screenshot of what I mean:

Here is a live demo and you can check the source code for more information as I wont be able to provide the proper code since I am not the original author of the style:
http://zirotec-uka.com/dugi/phpBB3/rules.php#rule1
Thank you.
You have a rule specifying
h3 { margin-top: 20px; }. That is what causes the space.You can override that with
#main h3 { margin-top: 0; }.