The reason why this happens is because Sitefinity Navigation uses the same classes for the divs and spans that wrap the individual pages. In order to change either the classes or the IDs if the navigation tabs you can go to Design->Widget Tenplates and either modify the existing widgets there or create a brand new one. That way you can style your navigation to your liking.
Ivan D. Dimitrov
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items