Editing W3C Markup Validation Service (section)

{{short description|Validator service by the World Wide Web Consortium}}
{{multiple image | direction = vertical | width = 40%
|image1=Valid XHTML 1.0.svg
|image2=Valid CSS.svg
|caption2=Tag certifying that a website has been checked for well-formed XHTML (above) and CSS (below) markup}}

The '''Markup Validation Service''' is a [[validator]] by the World Wide Web Consortium (W3C) that allows [[Internet]] users to check pre-HTML5 [[HTML]] and [[XHTML]] documents for [[Well-formed element|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.<ref name="w3.org_about">{{cite web |url=http://validator.w3.org/about.html |title=About the W3C Markup Validation Service |access-date=2011-05-20 |work=W3C Markup Validation Service |publisher=World Wide Web Consortium }}</ref>  Though W3C validation is important for browser compatibility and site usability, it has not been confirmed what effect it has on [[search engine optimization]].

As [[HTML5]] 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.<ref>{{cite web |title=Ready to check - Nu Html Checker |url=https://validator.w3.org/nu/ |website=validator.w3.org}}</ref>