HTML is an SGML-DTD, and XHTML is an
XML-DTD...
Yes, it's true. Anyone can write a DTD and those document
almost looks
like HTML....
In the document:
<?xml standalone="no" ?>
<!DOCTYPE
paper SYSTEM
paper.dtd>
<paper background="black">
<p>A paragraph...</p>
</paper>
In the DTD:
<!ELEMENT paper (p*, #PCDATA)>
<!ELEMENT p (#PCDATA)>
<!ATTRLIST paper
background (black|red|blue|
white)
>