Why Id's to elements must be unique?

[sfraire]

I know I should not ask this type of question here but I'm too discharged my mind googling the question so at last I've been asking you this, please don't view as negative:\[code\]<div id="full-width"><div id="wrap><!--contents with fixed width:: 950px--></div></div><div id="full-width"><div id="wrap><!--contents with fixed width:: 950px--></div></div><div id="wrap"><!--contents with fixed width:: 950px--></div><div id="wrap"><!--contents with fixed width:: 950px--></div><div id="full-width"><!--contents with full width:: 100%--></div>\[/code\]Key QuestionThis works fine, but many coders or say css rule directs that we should not use same id more than one times. So how could I place my markup? And could anyone describe about that we should use only 1 id not more than this which doesn't work?EditI want to use Id in main contents, so how could I implement this, just give me markup only, please.