An extensible editor allows integration of extensions that modify the editor's default behavior and provide customized feedback to users. The editor includes interfaces through which extensions are connected to the editor and through which selection services and highlight rendering services are provided. The selection services interfaces provide a clear separation of a logical selection position in the document and the visual feedback provided for the selection, allowing extensions to be designed that provide customized selection feedback. The highlight rendering services interfaces provide an extension with the ability to augment an existing selection without modifying the actual document. The editor also includes an event routing model that works to decrease the occurrence of conflicts between the editor and extensions and between extensions.