The Store Editor adds the following standard transitional DOCTYPE to most* published Store Editor pages:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">*Note: The DOCTYPE element is not added to the following pages:
There is no need to publish pages to add the DOCTYPE element. This element is added at the page-server level automatically. As such, you will not see the DOCTYPE element on pages within the Store Editor—only on published pages.
More information on DOCTYPES: