Extensible hypertext markup language (XHTML) validators are software programs that check the validity of the code of XHTML documents, sometimes loosely referred to simply as "web pages." Code is considered valid only if it adheres to the standards of the World Wide Web Consortium (W3C), an international community that sets standards to encourage continual growth and development of the Internet. XHTML validators not only check to see whether an XHTML document is written in valid code, it also generates a report that can be read by the people who use the software so they know whether the document is valid. A document type definition (DTD) is required as the first line of code in the file for a document to be validated. Most XHTML validators can locate documents to be validated by direct input, a path to the page or the uniform resource locator (URL) of the page.
Despite the knowledge or experience that a web developer has, he or she almost always makes at least a few mistakes when coding web documents in a simple text editor. If those documents contain a significant amount of code, it can be difficult and time consuming to manually review for errors. The reports that XHTML validators generate after checking validity of code provide very valuable information to end-users. Reports generally reveal not only errors, they also provide warnings and give hints for how to correct mistakes. Web designers and developers will know the exact line of code on which the warning or error was found because the line number of the invalid code is reported.
There are times when the line number of invalid code gives the developer only a good idea of where the problem is. This usually occurs when XHTML validators are given the URL of the document they are to check. If the document has been templated into a header, body and footer, line numbers indicating the location of errors will be based on the document as a whole, composed of all three parts. XHTML validators are widely used because of the need for web developers to produce standards-compliant XHTML documents. Cellular phones and personal digital assistants (PDA) figure among the media devices capable of displaying web pages, but they cannot always correctly display pages that are not written in valid XHTML code.
A variety of XHTML validators are available free of charge, but some require a fee. They all offer the basic functionality of verifying the validity of an XHTML document and reporting results to end-users, but some offer additional features that might prove helpful to some web developers, such as checking for broken hypertext links. It is important to keep in mind that XHTML validators should not be confused with software that handles the conversion of a hypertext markup language (HTML) document to XHTML.