XHTML Validation

An XHTML document can be validated with W3C's validator.

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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"