An XHTML document is validated against a Document Type Definition (DTD).
A proper DTD must be added as the first line of the file,before validating properly an XHTML file.
The Strict DTD includes all non-deprecated elements and attributes or
those which donot appear in framesets:
A D V E R T I S E M E N T
!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
The Transitional DTDcontains everything which the strict DTD includes
plus deprecated elements and attributes:
!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
The Frameset DTD includes frames and all the
other things which are included in transitional DTD:
!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Frameset//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd"
This is a simple XHTML document:
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd" >
<html>
<head>
<title>simple document</title>
</head>
<body>
<p>a simple paragraph</p>
</body>
</html>
