The doctype is used by , which are programs that examine document code to ensure that it meets all the syntax requirements of the specified language.

validators

Explanation:

The purpose of doctype is to inform the browser which version of markup language is being used. The doctype is used by validators which are programs that checks document code in order to make sure that it follows the correct syntax of the specified language. Markup Validator by W3C helps in determining the validity of the web documents. If doctype is omitted then browser will render the document in "quirks" mode rather than "standard" mode.

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