Valid markup does not necessarily make for 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.
More sophisticated validation checks the integrity of the information provided; that the postal address exists, the credit card has not expired, etc.
W3C stands for the World Wide Web Consortium who are the recognized standards organization for the world wide web.
Guidelines can include checkpoints that are subjective and cannot be evaluated programmatically.
For online shopping systems this information typically includes first and last name, postal- and billing addresses and credit card details.
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.
This organization was founded by Tim Berners-Lee and is run by a full-time staff to continue to develop and maintain web standards.
These standards are then used to help guide web developers and browsers to develop code that lives up to certain standards.
In a nutshell, they write the rule-book that helps to define if our code is well-written or poorly written.