Web standards are the formal, non-proprietary standards and other technical specifications that define and describe aspects of the World Wide Web. In recent years, the term has been more frequently associated with the trend of endorsing a set of standardized best practices for building web sites, and a philosophy of web design and development that includes those methods. Web standards include many interdependent standards and specifications, some of which govern aspects of the Internet, not just the World Wide Web. Even when not web-focused, such standards directly or indirectly affect the development and administration of web sites and web services. Considerations include the interoperability, accessibility and usability of web pages and web sites. Standards and "Living standards" published by the Web Hypertext Application Technology Working Group (WHATWG), such as the HTML Living Standard, DOM Standard, Encoding Standard and URL Standard. Web standards are evolving specifications of web technologies. Web standards are developed by standards organizations-groups of interested and often competing parties chartered with the task of standardization-not technologies developed and declared to be a standard by a single individual or company.

It is crucial to distinguish those specifications that are under development from the ones that already reached the final development status (in case of W3C specifications, the highest maturity level). The earliest visible manifestation of the web standards movement was the Web Standards Project, launched in August 1998 as a grassroots coalition fighting for improved web standards support in browsers. The web standards movement supports concepts of standards-based web design, including the separation of document structure from a web page or application's appearance and behavior; an emphasis on semantically structured content that validates (that is, contains no errors of structural composition) when tested against validation software maintained by the World Wide Web Consortium; and progressive enhancement, a layered approach to web page and application creation that enables all people and devices to access the content and functionality of a page, regardless of personal physical ability (accessibility), connection speed, and browser capability.

Prior to the web standards movement, many web page developers used invalid, incorrect HTML syntax such as "table layouts" and "spacer" GIF images to create web pages - an approach often referred to as "tag soup". Such pages sought to look the same in all browsers of a certain age (such as Microsoft Internet Explorer 4 and Netscape Navigator 4), but were often inaccessible to people with disabilities. Tag soup pages also displayed or operated incorrectly in older browsers, and required code forks such as JavaScript for Netscape Navigator and JScript for Internet Explorer that added to the cost and complexity of development. The extra code required, and the lack of a caching page layout language, made web sites "heavy" in terms of bandwidth, as did the frequent use of images as text. These bandwidth requirements were burdensome to users in developing countries, rural areas, and wherever fast Internet connections were unavailable.

The Web Standards movement pioneered by Glenn Davis, George Olsen, Jeffrey Zeldman, Steven Champeon, Todd Fahrner, Eric A. Meyer, Tantek Çelik, Dori Smith, Tim Bray, Jeffrey Veen, and other members of the Web Standards Project replaced bandwidth-heavy tag soup with light, semantic markup and progressive enhancement, with the goal of making web content "accessible to all". The Web Standards movement declared that HTML, CSS, and JavaScript were more than simply interesting technologies. The group succeeded in persuading Netscape, Microsoft, and other browser makers to support these standards in their browsers. It then set about promoting these standards to designers, who were still using tag soup, Adobe Flash, and other proprietary technologies to create web pages. In 2007, Douglas Vos initiated the Blue Beanie Day, inspired by Jeffrey Zeldman, who is shown with a blue cap on the book cover of his 2003 book Designing with Web Standards. Since then, the 30 November is the annual international celebration of web standards and web accessibility.

When a web site or web page is described as complying with web standards, it usually means that the site or page has valid HTML, CSS and JavaScript. The HTML should also meet accessibility and semantic guidelines. Full standard compliance also covers proper settings for character encoding, valid RSS or valid Atom news feed, valid RDF, valid metadata, valid XML, valid object embedding, valid script embedding, browser- and resolution-independent codes, and proper server settings. Recommendations for markup languages, such as Hypertext Markup Language (HTML), Extensible Hypertext Markup Language (XHTML), and Scalable Vector Graphics (SVG) from W3C. Recommendations for stylesheets, especially Cascading Style Sheets (CSS), from W3C. Standards for ECMAScript, more commonly JavaScript, from Ecma International. Recommendations for Document Object Models (DOM), from W3C. Properly formed names and addresses for the page and all other resources referenced from it (URIs), based upon RFC 2396, from IETF. Proper use of HTTP and MIME to deliver the page, return data from it and to request other resources referenced in it, based on RFC 2616, from IETF.

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