Validation is the process of checking data against a standard or requirement.
The term is commonly used when:
Webpage validation involves checking the code used to create a webpage (markup), against a computer-language specification.
A language specification sets out the parts of the language, and how the language can be used to write instructions that can be understood (processed or parsed) by a computer.
A number of the languages used to author webpages are formalised and extended by the World Wide Web Consortium (W3C).
Languages commonly used to write (author) webpages include:
Markup that conforms to a specification is described as ‘valid’ or ‘well-formed’.
Non-conforming markup is described as ‘tag-soup’, (as the underlying code often resembles a jumble of letters—à la alphabet soup).
Code validation is typically carried out using an online (automated) service or validator.
For example the W3C Markup validation service can be used to validate HTML and XHTML.
For a webpage to be validated:
The validation process:
Tip: The W3C validator can also be activated by linking to the service from the webpage to be validated:
<a href="http://validator.w3.org/check?uri=referer">Validate HTML</a>
Test this webpage: Validate HTML
Validate HTML is not required.)
An applied specification is a guide to creating web content for a specific user community.
For example the Web Accessibility Initiative (WAI), a working group of the W3C, has created Web Content Accessibility Guidelines (WCAG). These guidelines identify best-practices and techniques that enable people with disabilities equal-access to web content.
Compliance with an applied specification can also be evaluated. Cynthia is an online validator that identifies incorrect markup, additional markup requirements, and content that a webpage must contain to meet the WCAG checkpoints (and also United States disability law requirements as defined by Section 508).
It is important to note that automated applied-specification validation has its limitations. Guidelines can include checkpoints that are subjective and cannot be evaluated programmatically. For example, WCAG checkpoint 14.1:
14.1 Use the clearest and simplest language appropriate for a site’s content. 
Determining whether language used is ‘clear’, ‘simple’ or ‘appropriate’ can only be assessed by recruiting people who are familiar with the content domain, and who are also representative of the website’s intended audience.
Valid markup does not necessarily make for accessible web content.
Specification-based validation is comparable to a grammar-check.
Specification-based validation is comparable to a grammar-check. Although words (elements) may be spelt correctly (use the correct tags) and be drawn from the same language, without appropriate grammar (structure) they can be meaningless.
For content to be accessible, the author must markup content into elements that convey the meaning of the content (semantic-markup).
Semantic markup can be used as a ‘hook’ by assistive technologies that enable users with disabilities to access web content, for example:
Hkey can be used to move between headings to quickly scan the content of a webpage
mandatory or required to ensure a minimum level of accessibility, to a level 2:
considered to enhance accessibility).
Form validation ensures that the user has provided sufficient information for a transaction to be completed. For online shopping systems this information typically includes first and last name, postal- and billing addresses and credit card details.
More sophisticated validation checks the integrity of the information provided; that the postal address exists, the credit card has not expired, etc.