How is Visualforce Versioned?

Starting with the Summer '09 release, Visualforce pages and components are versioned. When a page or component has a version number, the functionality of older Visualforce elements does not change as new implementations are introduced. Visualforce versions start at 15.0. If you try to set the version of a Visualforce page to a version earlier than 15.0, it will automatically be changed to 15.0.

To aid backwards-compatibility, each Visualforce page and custom component is saved with version settings for the specified version of the API as well as the specific version of Visualforce. If the Visualforce page or component references installed managed packages, the version settings for each managed package referenced by the page or component is saved too. This ensures that as Visualforce, the API, and the components in managed packages evolve in subsequent versions, Visualforce pages and components are still bound to versions with specific, known behavior.

Custom components that are referenced in Visualforce pages always perform under their own version number. Thus, if a custom component is set at version 15.0, it always exhibits behavior from Visualforce version 15.0, whether running in a version 15.0 or a 16.0 page.

The release notes list any changes between Visualforce versions. The component reference also lists which Visualforce version a standard component was introduced in, as well as whether a component or attribute was deprecated in a version.