A kind request. Urgent

Hi,
 If I write some invalid dtd to a particular xml document, the browser is
not giving the error invalid document.
The xml document is as follows :
<?xml version="1.0" standalone="yes"?>
<!DOCTYPE family [
  <!ELEMENT parent (#PCDATA)>
  <!ELEMENT child (#PCDATA)>
]>

<family>
vafhsdfhsdh
<parent>Judy</parent>
<parent>Layard</parent>
<child>Jennifer</child>
<child>Brendan</child>
</family>

But if I write some internal entity, it is accepting it.
It is very urgent. I hope I will get a solution from you as soon as possible.
I checked it with IE5.5
Thanks in advance,
Leela.A