A validator is a software program that can check your web pages against the web standards.
When using a validator to check HTML, XHTML or CSS documents, the validator returns a list of errors found, according to your chosen standard.
Make sure you make it a habit to validate all your web pages before publishing.
An XHTML document is validated against a Document Type Definition (DTD).
You can read more about XHTML validation in our HTML tutorial.
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