Un document XHTML poate fi validat folosind validatoru-l W3C.

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Validarea declaratiilor DTD XHTML

Inainte ca un document XHTML sa fie corect validat, trebuie sa aibe prima linie o declaratie corecta DTD.

Declaratia DTD Strict include elemente si atribute la care nu s-a renuntat inca si care nu apar in frameset-uri:

!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"

Declaratia DTD Transitional include elementele DTD strict si elementele si atributele la care s-a renuntat:

!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"

Declaratia DTD Frameset include elementele din DTD transitional  plus elementele de frame-uri:

!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Frameset//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd"