Questions on the Extensibility of XHTML

I have some questions,

Firstly,

On the basis of Extensibility of XHTML, one can make his own XHTML family Document type.
But I wonder if His DTD should be accepted as a kind of specification for the world-wide 
usage of his DTD. (e.g, XHTML-Basic) 
Is it right ?

Secondly,

I know Some Web browsers support XHTML1.0, because XHTML1.0 has HTML tag sets.
While, How an extended XHTML document which has extended element can be shown 
on general web browser ?? 
If this DTD fragment has different namespace and doesn't have any presentational property,
How the web-browser handle this element??

Hope your kind advices.

Regards,

Jin.