The Markup Validation Service is a validator by the

World Wide Web Consortium The World Wide Web Consortium (W3C) is the main international standards organization for the World Wide Web. Founded in 1994 and led by Tim Berners-Lee, the consortium is made up of member organizations that maintain full-time staff working ...

(W3C) that allows

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users to check pre-HTML5

HTML The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaS ...

and

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

documents for well-formed markup against a

document type definition A document type definition (DTD) is a set of ''markup declarations'' that define a ''document type'' for an SGML-family markup language ( GML, SGML, XML, HTML). A DTD defines the valid building blocks of an XML document. It defines the document s ...

. Markup validation is an important step towards ensuring the technical quality of web pages. However, it is not a complete measure of

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conformance. Though W3C validation is important for browser compatibility and site usability, it has not been confirmed what effect it has on

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. As

HTML5 HTML5 is a markup language used for structuring and presenting content on the World Wide Web. It is the fifth and final major HTML version that is a World Wide Web Consortium (W3C) recommendation. The current specification is known as the HTML ...

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 Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation of a document written in a markup language such as HTML or XML (including XML dialects such as SVG, MathML or XHTML). CSS is a cornerstone technolo ...

, XML schemas, and

MathML Mathematical Markup Language (MathML) is a mathematical markup language, an application of XML for describing mathematical notations and capturing both its structure and content. It aims at integrating mathematical formulae into World Wide W ...

Browser accommodation

Many major web browsers 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 arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. T ...

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