When looking at the HTML 4 dtd (https://www.w3.org/TR/html4/sgml/dtd.html) I see a lot of descriptive comments within the dtd definitions.
<!ELEMENT A - - (%inline;)* -(A) -- anchor -->
<!ATTLIST A
%attrs; -- %coreattrs, %i18n, %events --
charset %Charset; #IMPLIED -- char encoding of linked resource --
type %ContentType; #IMPLIED -- advisory content type --
name CDATA #IMPLIED -- named link end --
href %URI; #IMPLIED -- URI for linked resource --
hreflang %LanguageCode; #IMPLIED -- language code --
rel %LinkTypes; #IMPLIED -- forward link types --
rev %LinkTypes; #IMPLIED -- reverse link types --
accesskey %Character; #IMPLIED -- accessibility key character --
shape %Shape; rect -- for use with client-side image maps --
coords %Coords; #IMPLIED -- for use with client-side image maps --
tabindex NUMBER #IMPLIED -- position in tabbing order --
onfocus %Script; #IMPLIED -- the element got the focus --
onblur %Script; #IMPLIED -- the element lost the focus --
>
I am aware that this is an SGML dtd. When I try to introduce them in my XML dtds, I get parsing errors from the processor. Have thesecomments been removed from the DTD variant for XML?
post.dtd:36: parser error : expected '>'
<!ELEMENT A (#PCDATA) -- a link -- >
^
post.dtd:36: parser error : Content error in the external subset
<!ELEMENT A (#PCDATA) -- a link -- >
In SGML, comments starting with and ending in -- can appear anywhere, or multiple times in a markup declaration; in XML, a markup declaration must either contain just a single comment, or another markup declaration:
<!-- valid in XML -->
<!-- only -- -- valid -- -- in -- -- SGML -->
As a consequence of XML being defined as an SGML subset, the text string -- isn't allowed to appear in XML comments anywhere.