One should specify all of certain groups of related properties together. "Together" means a number of things:
These groups of related properties are any groups of properties that interact in a way that their being set from different stylesheets could make the document unreadable or ugly.
The most important such group of properties relates to colors and backgrounds. In certain cases, one may also want to specify all of the box properties, although in other cases it may be acceptable only to give the right one.
(Up to User Stylesheets Guide, CSS, David Baron)
LDB, email@example.com, 1999-08-11