An ignorant SGML question

Michael Johnson (
Mon, 9 Jan 1995 21:57:02 +0100

Pardon my ignorance on this, but what is the effect of specifying 'O' instead
of '-' immediately following an element name? The BODY and HEAD elements in
the HTML3 DTD are defined this way:

<!ELEMENT BODY O O %body.content>
<!ELEMENT HEAD O O (%head.content)>

I've been able to interpret other DTD constructs by context and knowledge of
regular expressions, but this one eludes me. If I had to guess, I'd guess that
it means you can omit the end tag, but that the tag is not implicitly closed
by an element that it cannot contain.

I'd like to know for sure, though.

Michael Johnson
Relay Technology, Inc.