The Markup Validation Service is a

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by the World Wide Web Consortium (W3C) that allows

Internet The Internet (or internet) is the Global network, global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a internetworking, network of networks ...

users to check pre-HTML5

HTML Hypertext Markup Language (HTML) is the standard markup language for documents designed to be displayed in a web browser. It defines the content and structure of web content. It is often assisted by technologies such as Cascading Style Sheets ( ...

and

XHTML Extensible HyperText Markup Language (XHTML) is part of the family of XML markup languages which mirrors or extends versions of the widely used HyperText Markup Language (HTML), the language in which Web pages are formulated. While HTML, pr ...

documents for well-formed markup against a document type definition (DTD). Markup validation is an important step towards ensuring the technical quality of web pages. However, it is not a complete measure of web standards conformance. Though W3C validation is important for browser compatibility and site usability, it has not been confirmed what effect it has on

search engine optimization Search engine optimization (SEO) is the process of improving the quality and quantity of Web traffic, website traffic to a website or a web page from web search engine, search engines. SEO targets unpaid search traffic (usually referred to as ...

. As

HTML5 HTML5 (Hypertext Markup Language 5) is a markup language used for structuring and presenting hypertext documents on the World Wide Web. It was the fifth and final major HTML version that is now a retired World Wide Web Consortium (W3C) recommend ...

has removed the use of DTD in favor of a "Living Standard", the traditional Markup Validation Service is not applicable to these formats. Validation is instead performed using an open-source "Nu Validator", an instance of which is provided by W3C.

History

The Markup Validation Service began as ''The Kinder, Gentler HTML Validator'', a project by Gerald Oskoboiny. It was developed to be a more intuitive version of the first online HTML validator written by Dan Connolly and Mark Gaither, which was announced on July 13, 1994. In September 1997, Oskoboiny began working for the W3C, and on December 18, 1997, the W3C announced its ''W3C HTML Validator'' based upon his works. In November 2008, the W3C released the validator.nu HTML5 engine and the ability to check documents for conformance to HTML5. W3C also offers validation tools for web technologies other than HTML/XHTML, such as CSS,

XML schemas An XML schema is a description of a type of Extensible Markup Language, XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntactical constraints imposed b ...

, and

MathML Mathematical Markup Language (MathML) is a pair of mathematical markup languages, an application of XML for describing mathematical notations and capturing both its structure and content. Its aim is to natively integrate mathematical formulae ...

Browser accommodation

Many major

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are often tolerant of certain types of error, and may display a document successfully even if it is not syntactically correct. Certain other

XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing data. It defines a set of rules for encoding electronic document, documents in a format that is both human-readable and Machine-r ...

documents can also be validated if they refer to an internal or external DTD.

Limitations

Mark-up validators cannot see the "big picture" on a web page, but they excel at picking up missed closing tags and other technicalities.Castro, Elisabeth: ''HTML, XHTML & CSS, Sixth Edition'', page 345–346. Visual Quickstart Guides, Peachpit Press, 2007. DTD-based validators are also limited in their ability to check attribute values according to many specification documents. For example, using an HTML 4.01 DOCTYPE, bgcolor="fffff" is accepted as valid for the "body" element even though the value "fffff" is missing a preceding '#' character and contains only five (instead of six) hexadecimal digits. Also, for the "img" element, width="really wide" is also accepted as valid. DTD-based validators are technically not able to test for these types of attribute value problems. Pages may not display as intended in all browsers, even in the absence of validation errors and successful display in other browsers. The only way to ensure that pages always display as intended is to test them in all browsers expected to render them correctly.

CSS validation

While the W3C and other HTML and XHTML validators will assess pages coded in those formats, a separate validator like the W3C CSS validator can check that there are no errors in the associated Cascading Style Sheet. CSS validators apply current CSS standards to referenced CSS documents.

References

External links