Is the HTML 4.01 DTD at www.w3.org supposed to be a valid DTD?

The element declarations in there are a bit unfamiliar. Here are two of them, for example:
<!ELEMENT HR - O EMPTY -- horizontal rule -->
<!ELEMENT Q - - (%inline;)* >

The problem areas are underlined. What do the "- -" and the "- O" mean and are they really supposed to be there?

Also, are there really supposed to be comments in the tag like that?