With Content Workflow on, create a new HTML feature, change the settings to display Modified By and Modified Date, edit the content, and Create Draft. The Modified By property is blank; we get "Modified 17/07/2013 by ".
When the content is published, the property displays, but when set back to draft it disappears again.
Replicated on the demo site at https://demo.mojoportal.com/test2
(The Created By property works fine).