CSS classes in SharePoint 2010 follow CSS standards, including styles of type ID-based, class-based, and element-based styles. The following shows you how SharePoint uses each type of style and provides examples.
ID-based styles are prefixed by a # symbol. An ID-based style is unique and can only be applied to a page once. If you create duplicate IDs then the first instance of the ID on the page typically overrides subsequent duplicate entries. For instance, the v4.master page already contains an ID of
s4-workspace. In you insert a new DIV element above the existing s4-workspace ID with the same ID then the content of the new ID overrides the default
Examples of ID-based styles in ...