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Validate XHTML With A DTD
Before an XHTML file can be validated, a correct DTD must be added as the first line of the file.
The Strict DTD includes elements and attributes that have not been deprecated or do not appear in framesets:
!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
The Transitional DTD includes everything in the strict DTD 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 everything in the transitional DTD plus frames as well:
!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd"
