The various computer languages used on the Web, including but not limited to (X)HTML, CSS, RDF, and RSS, provide structure, style, metadata, semantics, and other document features. Similar to natural languages, they have their own grammar, vocabulary, and syntax that need to be followed. However, just like the grammar, structural, or spelling errors that occur in documents written in natural languages, web documents might also have errors in them. Validation is the task of checking the source code of web documents against a DTD or schema. It contributes to error-free, clean code and increases overall web page quality.

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