Parser&DTD....

["AmirHossein GholamiPour" <gholamipour@ce.sharif.edu>]

Hello to everybody
Dear All
I don't know much about a xml parser and what it is doing and how I can
write a parser for myself,and also when do we call a xml document invalid.
I mean we call it valid in relation to its DTD but when it is invalid.
One more question,excuse me,what is a Style Sheet,and CSS?
Thank You Very Much!
Be Successful!