dtd-start condition*
     action*

A rule performed at the start of a DTD, immediately after the document element name is determined, so that the DTD's "name" is known. (The #doctype built-in stream can be used to return the DTD's name.)

The fact that the document element name is known means that any comment between the doctype keyword and the document element name cannot be distinguished from those prior to the DTD. dtd-start allows comments and processing instructions prior to the DTD to be distinguished from those within the DTD.