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According W3C XML specification, element is declared in form of :
[45] elementdecl ::= '<!ELEMENT' S Name S contentspec S? '>'
[46] contentspec ::= 'EMPTY' | 'ANY' | Mixed | children
And samples on w3school is :
<!ELEMENT br EMPTY>
But in html 4.01 strict dtd element br is ：
<!ELEMENT BR - O EMPTY -- forced line break -->
Why the second declaration add “- O”? What it means? And Where can I find the offical document about this?
The hyphen means required, the O means optional. So “- O” means that the opening tag is required, and the closing tag is optional. You can read more in §3.3.3